Thealternatingstrataofporphyriesandporphyriticconglomerate，andwiththeoccasionallyincludedbedsoffeldspathicslate，togethermakeagrandformation;inseveralplaceswithintheCordillera，Iestimateditsthicknessatfromsixtoseventhousandfeet。Itextendsformanyhundredmiles，formingthewesternflankoftheChileanCordillera;andevenatIquiqueinPeru，850milesnorthofthesouthernmostpointexaminedbymeinChile，thecoast-escarpmentwhichrisestoaheightofbetweentwoandthreethousandfeetisthuscomposed。InseveralpartsofNorthernChilethisformationextendsmuchfurthertowardsthePacific，overthegraniticandmetamorphiclowerrocks，thanitdoesinCentralChile;butthemainCordilleramaybeconsideredasitscentralline，anditsbreadthinaneastandwestdirectionisnevergreat。Atfirsttheoriginofthisthick，massive，longbutnarrowformation，appearedtomeveryanomalous：whencewerederived，andhowweredispersedtheinnumerablefragments，oftenoflargesize，sometimesangularandsometimesrounded，andalmostinvariablycomposedofporphyriticrocks？Seeingthattheinterstratifiedporphyriesarenevervesicularandoftennotevenamygdaloidal，wemustconcludethatthepilewasformedindeepwater;howthencamesomanyfragmentstobewellroundedandsomanytoremainangular，sometimesthetwokindsbeingequallymingled，sometimesoneandsometimestheotherpreponderating？Thattheclaystone，greenstone，andotherporphyriesandamygdaloids，whichlieCONFORMABLYbetweenthebedsofconglomerate，areancientsubmarinelavas，Ithinktherecanbenodoubt;andIbelievewemustlooktothecraterswhencethesestreamswereerupted，asthesourceofthebreccia-
conglomerate;afterthegreatexplosion，wemayfairlyimaginethatthewaterintheheatedandscarcelyquiescentcraterwouldremainforaconsiderabletimesufficientlyagitatedtotriturateandroundtheloosefragments，fewormanyinnumber，wouldbeshotforthatthenexteruption，associatedwithfewormanyangularfragments，accordingtothestrengthoftheexplosion。(Thiscertainlyseemstohavetakenplaceinsomerecentvolcanicarchipelagos，asattheGalapagos，wherenumerouscratersareexclusivelyformedoftuffandfragmentsoflava。)Theporphyriticconglomeratebeingpurpleorreddish，evenwhenalternatingwithdusty-
colouredorbrightgreenporphyriesandamygdaloids，isprobablyananalogouscircumstancetothescoriaeoftheblackishbasaltsbeingoftenbrightred。Theancientsubmarineorificeswhencetheporphyriesandtheirfragmentswereejectedhavingbeenarrangedinaband，likemoststillactivevolcanoes，accountsforthethickness，thenarrowness，andlinearextensionofthisformation。
Thiswholegreatpileofrockhassufferedmuchmetamorphicaction，asisveryobviousinthegradualformationandappearanceofthecrystalsofalbiticfeldsparandofepidote——inthebendingtogetherofthefragments——
intheappearanceofalaminatedstructureinthefeldspathicslate——and，lastly，inthedisappearanceoftheplanesofstratification，whichcouldsometimesbeseenonthesamemountainquitedistinctintheupperpart，lessandlessplainontheflanks，andquiteobliteratedatthebase。
Partlyowingtothismetamorphicaction，andpartlytothecloserelationshipinorigin，Ihaveseenfragmentsofporphyries——takenfromametamorphosedconglomerate——fromaneighbouringstreamoflava——fromthenucleusorcentre(asitappearedtome)ofthewholesubmarinevolcano——
andlastlyfromanintrusivemassofquitesubsequentorigin，allofwhichwereabsolutelyundistinguishableinexternalcharacters。
Oneotherrock，ofplutonicorigin，andhighlyimportantinthehistoryoftheCordillera，fromhavingbeeninjectedinmostofthegreataxesofelevation，andfromhavingapparentlybeeninstrumentalinmetamorphosingthesuperincumbentstrata，maybeconvenientlydescribedinthispreliminarydiscussion。IthasbeencalledbysomeauthorsANDESITE：itmainlyconsistsofwell-crystallisedwhitealbite(asdeterminedwiththegoniometerinnumerousspecimensbothbyProfessorMillerandmyself)，oflessperfectlycrystallisedgreenhornblende，oftenassociatedwithmuchmica，withchloriteandepidote，andoccasionallywithafewgrainsofquartz：inoneinstanceinNorthernChile，Ifoundcrystalsoforthiticorpotashfeldspar，mingledwiththoseofalbite。(Ihere，andelsewhere，callbythisname，thosefeldspathicmineralswhichcleavelikealbite：butitnowappears(“EdinburghNewPhilosophicalJournal“volume24page181)thatAbichhasanalysedamineralfromtheCordillera，associatedwithhornblendeandquartz(probablythesamerockwiththathereunderdiscussion)，whichcleaveslikealbite，butwhichisanewanddistinctkind，calledbyhimANDESINE。Itisalliedtoleucite，withthegreaterproportionofitspotashreplacedbylimeandsoda。Thismineralseemsscarcelydistinguishablefromalbite，exceptbyanalysis。)Wherethemicaandquartzareabundant，therockcannotbedistinguishedfromgranite;anditmaybecalledandesiticgranite。Wherethesetwomineralsarequiteabsent，andwhen，asoftenthenhappens，thecrystalsofalbiteareimperfectandblendtogether，therockmaybecalledandesiticporphyry，whichbearsnearlythesamerelationtoandesiticgranitethateuriticporphyrydoestocommongranite。Theseandesiticrocksformmountainmassesofawhitecolour，which，intheirgeneraloutlineandappearance——intheirjoints——intheiroccasionallyincludingdark-coloured，angularfragments，apparentlyofsomepre-existingrock——andinthegreatdikesbranchingfromthemintothesuperincumbentstrata，manifestacloseandstrikingresemblancetomassesofcommongraniteandsyenite：Inever，however，sawintheseandesiticrocks，thosegraniticveinsofsegregationwhicharesocommonintruegranites。WehaveseenthatandesiteoccursinthreeplacesinTierradelFuego;inChile，fromS。FernandotoCopiapo，adistanceof450miles，Ifounditundermostoftheaxesofelevation;inacollectionofspecimensfromtheCordilleraofLimainPeru，Iimmediatelyrecognisedit;andErmanstatesthatitoccursinEasternKamtschatka。(“GeographicalJournal“volume9page510。)Fromitswiderange，andfromtheimportantpartithasplayedinthehistoryoftheCordillera，IthinkthisrockhaswelldeserveditsdistinctnameofAndesite。
ThefewstillactivevolcanoesinChileareconfinedtothecentralandloftiestrangesoftheCordillera;andvolcanicmatter，suchasappearstohavebeenofsubaerialeruption，iseverywhererare。AccordingtoMeyen，thereisahillofpumicehighupthevalleyoftheMaypu，andlikewiseatrachyticformationatColina，avillagesituatednorthofSt。Jago。
(“ReiseumErde“Th。1ss。338and362。)Closetothislattercity，therearetwohillsformedofapalefeldspathicporphyry，remarkablefrombeingdoublycolumnar，greatcylindricalcolumnsbeingsubdividedintosmallerfour-orfive-sidedones;andathirdhillock(CerroBlanco)isformedofafragmentarymassofrock，whichIbelievedtobeofvolcanicorigin，intermediateincharacterbetweentheabovefeldspathicporphyryandcommontrachyte，andcontainingneedlesofhornblendeandgranularoxideofiron。
NeartheBathsofCauquenes，betweentwoshortparallellinesofelevation，wheretheyareintersectedbythevalley，thereisasmall，thoughdistinctvolcanicdistrict;therockisadarkgrey(andesitic)trachyte，whichfusesintoagreenish-greybead，andisformedoflongcrystalsoffracturedglassyalbite(judgingfromonemeasurement)mingledwithwell-
formedcrystals，oftentwin，ofaugite。Thewholemassisvesicular，butthesurfaceisdarkercolouredandmuchmorevesicularthananyotherpart。
Thistrachyteformsacliff-bounded，horizontal，narrowstriponthesteepsouthernsideofthevalley，attheheightoffourorfivehundredfeetabovetheriver-bed;judgingfromanapparentlycorrespondinglineofcliffonthenorthernside，thevalleymustoncehavebeenfilleduptothisheightbyafieldoflava。OnthesummitofaloftymountainsomeleagueshigherupthissamevalleyoftheCachapual，Ifoundcolumnarpitchstoneporphyriticwithfeldspar;Idonotsupposethisrocktobeofvolcanicorigin，andonlymentionithere，fromitsbeingintersectedbymassesanddikesofaVESICULARrock，approachingincharactertotrachyte;innootherpartofChiledidIobservevesicularoramygdaloidaldikes，thoughthesearesocommoninordinaryvolcanicdistricts。
PASSAGEOFTHEANDESBYTHEPORTILLOORPEQUENESPASS。
AlthoughIcrossedtheCordilleraonlyoncebythispass，andonlyoncebythatoftheCumbreorUspallata(presentlytobedescribed)，ridingslowlyandhaltingoccasionallytoascendthemountains，therearemanycircumstancesfavourabletoobtainingamorefaithfulsketchoftheirstructurethanwouldatfirstbethoughtpossiblefromsoshortanexamination。Themountainsaresteepandabsolutelybareofvegetation;theatmosphereisresplendentlyclear;thestratificationdistinct;andtherocksbrightlyandvariouslycoloured：someofthenaturalsectionsmightbetrulycomparedfordistinctnesstothosecolouredonesingeologicalworks。Consideringhowlittleisknownofthestructureofthisgiganticrange，towhichIparticularlyattended，mosttravellershavingcollectedonlyspecimensoftherocks，Ithinkmysketch-sections，thoughnecessarilyimperfect，possesssomeinterest。Section1/1inPlate1whichIwillnowdescribeindetail，isonahorizontalscaleofathirdofaninchtoanauticalmile，andonaverticalscaleofoneinchtoamile(or6，000
feet)。Thewidthoftherange(excludingafewoutlyinghillocks)，fromtheplainonwhichSt。JagothecapitalofChilestands，tothePampas，issixtymiles，asfarasIcanjudgefromthemaps，whichdifferfromeachotherandareallEXCEEDINGLYimperfect。TheSt。JagoplainatthemouthoftheMaypu，Iestimatefromadjoiningknownpointsat2，300feet，andthePampasat3，500feet，bothabovethelevelofthesea。TheheightofthePequenesline，accordingtoDr。Gillies，is13，210feet(“JournalofNaturalandGeographicalScience“August1830。);andthatofthePortilloline(bothinthegapswheretheroadcrossesthem)is14，345feet;thelowestpartoftheintermediatevalleyofTenuyanis7，530feet——allabovethelevelofthesea。
TheCordillerahere，andindeedIbelievethroughoutChile，consistofseveralparallel，anticlinalanduniclinalmountain-lines，rangingnorth，ornorthwithalittlewesting，andsouth。Someexteriorandmuchlowerridgesoftenvaryconsiderablyfromthiscourse，projectinglikeobliquespursfromthemainranges：inthedistricttowardsthePacific，themountains，asbeforeremarked，extendinvariousdirections，eveneastandwest。Inthemainexteriorlines，thestrata，asalsobeforeremarked，areseldominclinedatahighangle;butinthecentralloftyridgestheyarealmostalwayshighlyinclined，brokenbymanygreatfaults，andoftenvertical。AsfarasIcouldjudge，fewoftherangesareofgreatlength：
andinthecentralpartsoftheCordillera，Iwasfrequentlyabletofollowwithmyeyearidgegraduallybecominghigherandhigher，asthestratificationincreasedininclination，fromoneendwhereitsheightwastriflinganditsstratagentlyinclinedtotheotherendwhereverticalstrataformedsnow-cladpinnacles。EvenoutsidethemainCordillera，nearthebathsofCauquenes，Iobservedonesuchcase，whereanorthandsouthridgehaditsstratainthevalleyinclinedat37degrees，andlessthanamilesouthofitat67degrees：anotherparallelandsimilarlyinclinedridgeroseatthedistanceofaboutfivemiles，intoaloftymountainwithabsolutelyverticalstrata。WithintheCordillera，theheightoftheridgesandtheinclinationofthestrataoftenbecamedoubledandtrebledinmuchshorterdistancesthanfivemiles;thispeculiarformofupheavalprobablyindicatesthatthestratifiedcrustwasthin，andhenceyieldedtotheunderlyingintrusivemassesunequally，atcertainpointsonthelinesoffissure。
Thevalleys，bywhichtheCordilleraaredrained，followtheanticlinalorrarelysynclinaltroughs，whichdeviatemostfromtheusualnorthandsouthcourse;orstillmorecommonlythoselinesoffaultsorofunequalcurvature(thatis，lineswiththestrataonbothhandsdippinginthesamedirection，butatasomewhatdifferentangle)whichdeviatemostfromanortherlycourse。Occasionallythetorrentsrunforsomedistanceinthenorthandsouthvalleys，andthenrecovertheireasternorwesterncoursebyburstingthroughtherangesatthosepointswherethestratahavebeenleastinclinedandtheheightconsequentlyisless。Hencethevalleys，alongwhichtheroadsrun，aregenerallyzigzag;and，indrawinganeastandwestsection，itisnecessarytocontractgreatlythatwhichisactuallyseenontheroad。
CommencingatthewesternendofSection1/1wheretheR。MaypudebouchesontheplainofSt。Jago，weimmediatelyenterontheporphyriticconglomerateformation，andinthemidstofitfindsomehummocks[A]ofgraniteandsyenite，whichprobably(forIneglectedtocollectspecimens)
belongtotheandesiticclass。Thesearesucceededbysomeruggedhills[B]
ofdark-green，crystalline，feldspathicandinsomepartsslatyrocks，whichIbelievebelongtothealteredclay-slateformation。Fromthispoint，greatmountainsofpurplishandgreenish，generallythinlystratified，highlyporphyriticconglomerates，includingmanystrataofamygdaloidalandgreenstoneporphyries，extendupthevalleytothejunctionoftheriversYesoandVolcan。Asthevalleyhererunsinaverysoutherlycourse，thewidthoftheporphyriticconglomerateformationisquiteconjectural;andfromthesamecause，Iwasunabletomakeoutmuchaboutthestratification。Inmostoftheexteriormountainsthedipwasgentleanddirectedinwards;andatonlyonespotIobservedaninclinationashighas50degrees。NearthejunctionoftheR。Coloradowiththemainstream，thereisahillofwhitish，brecciated，partiallydecomposedfeldspathicporphyry，havingavolcanicaspectbutnotbeingreallyofthatnature：atTolla，however，inthisvalley，Dr。Meyenmetwithahillofpumicecontainingmica。(“ReiseumErde“Th。1ss。338，341。)AtthejunctionoftheYesoandVolcan[D]thereisanextensivemass，inwhiteconicalhillocks，ofandesite，containingsomemica，andpassingeitherintoandesiticgranite，orintoaspotted，semi-granularmixtureofalbitic(？)feldsparandhornblende：inthemidstofthisformationDr。Meyenfoundtruetrachyte。Theandesiteiscoveredbystrataofdark-coloured，crystalline，obscurelyporphyriticrocks，andabovethembytheordinaryporphyriticconglomerates，——thestrataalldippingawayatasmallanglefromtheunderlyingmass。Thesurroundingloftymountainsappeartobeentirelycomposedoftheporphyriticconglomerate，andIestimateditsthicknesshereatbetweensixandseventhousandfeet。
BeyondthejunctionoftheYesoandVolcan，theporphyriticstrataappeartodiptowardsthehillocksofandesiteatanangleof40degrees;butatsomedistantpointsonthesameridgetheyarebentupandvertical。
FollowingthevalleyoftheYeso，trendingN。E。(andthereforestillunfavourableforourtransversesection)，thesameporphyriticconglomerateformationisprolongedtoneartheCuestadelIndio，situatedatthewesternendofthebasin(likeadrainedlake)ofYeso。Somewaybeforearrivingatthispoint，distantloftypinnaclescappedbycolouredstratabelongingtothegreatgypseousformationcouldfirstbeseen。FromthesummitoftheCuesta，lookingsouthward，thereisamagnificentsectionalviewofamountain-mass，atleast2，000feetinthickness[E]，offineandesitegranite(containingmuchblackmica，alittlechloriteandquartz)，whichsendsgreatwhitedikesfarintothesuperincumbent，dark-coloured，porphyriticconglomerates。Atthelineofjunctionthetwoformationsarewonderfullyinterlacedtogether：inthelowerpartoftheporphyriticconglomerate，thestratificationhasbeenquiteobliterated，whilstintheupperpartitisverydistinct，thebedscomposingthecrestsofthesurroundingmountainsbeinginclinedatanglesofbetween70and80
degrees，andsomebeingevenvertical。Onthenorthernsideofthevalley，thereisagreatcorrespondingmassofandesiticgranite，whichisencasedbyporphyriticconglomerate，dippingbothonthewesternandeasternsides，atabout80degreestowest，butontheeasternsidewiththetipsofthestratabentinsuchamanner，astorenderitprobablethatthewholemasshasbeenonthatsidethrownoverandinverted。
InthevalleybasinoftheYeso，whichIestimatedat7，000feetabovethelevelofthesea，wefirstreachat[F]thegypseousformation。Itsthicknessisverygreat。Itconsistsinmostpartsofsnow-white，hard，compactgypsum，whichbreakswithasaccharinefracture，havingtranslucentedges;undertheblowpipegivesoutmuchvapour;itfrequentlyincludesnestsandexceedinglythinlayersofcrystallised，blackishcarbonateoflime。Large，irregularlyshapedconcretions(externallystillexhibitinglinesofaqueousdeposition)ofblackish-grey，butsometimeswhite，coarselyandbrilliantlycrystallised，hardanhydrite，aboundwithinthecommongypsum。Hillocks，formedofthehardestandpurestvarietiesofthewhitegypsum，standupabovethesurroundingparts，andhavetheirsurfacescrackedandmarked，justlikenewlybakedbread。Thereismuchpalebrown，softargillaceousgypsum;andthereweresomeintercalatedgreenbedswhichIhadnottimetoreach。Isawonlyonefragmentofseleniteortransparentgypsum，andthatperhapsmayhavecomefromsomesubsequentlyformedvein。
>Fromthemineralogicalcharactersheregiven，itisprobablethatthesegypseousbedshaveundergonesomemetamorphicaction。Thestrataaremuchhiddenbydetritus，buttheyappearedinmostpartstobehighlyinclined;
andinanadjoiningloftypinnacletheycouldbedistinctlyseenbendingup，andbecomingvertical，conformablywiththeunderlyingporphyriticconglomerate。Inverymanypartsofthegreatmountain-face[F]，composedofthingypseousbeds，therewereinnumerablemasses，irregularlyshapedandnotlikedikes，yetwithwell-definededges，ofanimperfectlygranular，palegreenish，oryellowish-whiterock，essentiallycomposedoffeldspar，withalittlechloriteorhornblende，epidote，iron-pyrites，andferruginouspowder：Ibelievethatthesecurioustrappeanmasseshavebeeninjectedfromthenotfardistantmountain-mass[E]ofandesitewhilststillfluid，andthatowingtothesoftnessofthegypseousstratatheyhavenotacquiredtheordinaryformsofdikes。Subsequentlytotheinjectionofthesefeldspathicrocks，agreatdislocationhastakenplace;
andthemuchshatteredgypseousstratahereoverlieahillock[G]，composedofverticalstrataofimpurelimestoneandofblackhighlycalcareousshaleincludingthreadsofgypsum：theserocks，asweshallpresentlysee，belongtotheupperpartsofthegypseousseries，andhencemustherehavebeenthrowndownbyavastfault。
Proceedingupthevalley-basinoftheYeso，andtakingoursectionsometimesononehandandsometimesontheother，wecometoagreathillofstratifiedporphyriticconglomerate[H]dippingat45degreestothewest;andafewhundredyardsfartheron，wehaveabedbetweenthreeorfourhundredfeetthickofgypsum[I]dippingeastwardataveryhighangle：herethenwehaveafaultandanticlinalaxis。Ontheoppositesideofthevalley，averticalmassofredconglomerate，conformablyunderlyingthegypsum，appearsgraduallytoloseitsstratificationandpassesintoamountainofporphyry。Thegypsum[I]iscoveredbyabed[K]，atleast1，000feetinthickness，ofapurplish-red，compact，heavy，fine-grainedsandstoneormudstone，whichfuseseasilyintoawhiteenamel，andisseenunderalenstocontaintrituratedcrystals。Thisissucceededbyabed[L]，1，000feetthick(IbelieveIunderstatethethickness)ofgypsum，exactlylikethebedsbeforedescribed;andthisagainiscappedbyanothergreatbed[M]ofpurplish-redsandstone。Allthesestratadipeastward;buttheinclinationbecomeslessandless，asweleavethefirstandalmostverticalbed[I]ofgypsum。
Leavingthebasin-plainofYeso，theroadrapidlyascends，passingbymountainscomposedofthegypseousandassociatedbeds，withtheirstratificationgreatlydisturbedandthereforenoteasilyintelligible：
hencethispartofthesectionhasbeenleftuncoloured。ShortlybeforereachingthegreatPequenesridge，theloweststratumvisible[N]isaredsandstoneormudstone，cappedbyavastthicknessofblack，compact，calcareous，shalyrock[O]，whichhasbeenthrownintofourlofty，thoughsmallridges：lookingnorthward，thestrataintheseridgesareseengraduallytoriseininclination，becominginsomedistantpinnaclesabsolutelyvertical。
TheridgeofPequenes，whichdividesthewatersflowingintothePacificandAtlanticOceans，extendsinanearlyN。N。W。andS。S。E。line;itsstratadipeastwardatanangleofbetween30and45degrees，butinthehigherpeaksbendingupandbecomingalmostvertical。Wheretheroadcrossesthisrange，theheightis13，210feetabovethesea-level，andIestimatedtheneighbouringpinnaclesatfromfourteentofifteenthousandfeet。Theloweststratumvisibleinthisridgeisaredstratifiedsandstone[P];onitaresuperimposedtwogreatmasses[QandS]ofblack，hard，compact，evenhavingaconchoidalfracture，calcareous，moreorlesslaminatedshale，passingintolimestone：thisrockcontainsorganicremains，presentlytobeenumerated。Thecompactervarietiesfuseeasilyinawhiteglass;andthisImayaddisaverygeneralcharacterwithallthesedimentarybedsintheCordillera：althoughthisrockwhenbrokenisgenerallyquiteblack，iteverywhereweathersintoanash-greytint。
Betweenthesetwogreatmasses[QandS]，abed[R]ofgypsumisinterposed，aboutthreehundredfeetinthickness，andhavingthesamecharactersasheretoforedescribed。Iestimatedthetotalthicknessofthesethreebeds[Q，R，S]atnearlythreethousandfeet;andtothismustbeadded，aswillbeimmediatelyseen，agreatoverlyingmassofredsandstone。
Indescendingtheeasternslopeofthisgreatcentralrange，thestrata，whichintheupperpartdipeastwardataboutanangleof40degrees，becomemoreandmorecurved，tilltheyarenearlyvertical;andalittlefurtheronwardsthereisseenonthefurthersideofaravine，athickmassofstrataofbrightredsandstone[T]，withtheirupperextremitiesslightlycurved，showingthattheywereonceconformablyprolongedoverthebeds[S]：onthesouthernandoppositesideoftheroad，thisredsandstoneandtheunderlyingblackshalyrocksstandvertical，andinactualjuxtaposition。Continuingtodescend，wecometoasynclinalvalleyfilledwithrubbish，beyondwhichwehavetheredsandstone[T2]correspondingwith[T]，andnowdipping，asisseenbothnorthandsouthoftheroad，at45degreestothewest;andunderit，thebeds[S2，R2，Q2，andIbelieveP2]incorrespondingorderandofsimilarcomposition，withthoseonthewesternflankofthePequenesrange，butdippingwestward。Closetothesynclinalvalleythedipofthesestratais45degrees，butattheeasternorfartherendoftheseriesitincreasesto60degrees。Herethegreatgypseousformationabruptlyterminates，andissucceededeastwardbyapileofmoremodernstrata。Consideringhowviolentlythesecentralrangeshavebeendislocated，andhowverynumerousdikesareintheexteriorandlowerpartsoftheCordillera，itisremarkablethatIdidnotherenoticeasingledike。Theprevailingrockinthisneighbourhoodistheblack，calcareous，compactshale，whilstinthevalley-basinoftheYesothepurplishredsandstoneormudstonepredominates，——bothbeingassociatedwithgypseousstrataofexactlythesamenature。Itwouldbeverydifficulttoascertaintherelativesuperpositionoftheseseveralmasses，forweshallafterwardsseeintheCumbrePassthatthegypseousandintercalatedbedsarelens-shaped，andthattheythinout，evenwhereverythick，anddisappearinshorthorizontaldistances：itisquitepossiblethattheblackshalesandredsandstonesmaybecontemporaneous，butitismoreprobablethattheformercomposetheuppermostpartsoftheseries。
Thefossilsabovealludedtointheblackcalcareousshalesarefewinnumber，andareinanimperfectcondition;theyconsist，asnamedformebyM。d’Orbigny，of：——
1。Ammonite，indeterminable，neartoA。recticostatus，d’Orbigny，“Pal。
Franc。”(Neocomianformation)。
2。Gryphaea，neartoG。Couloni(NeocomianformationsofFranceandNeufchatel)。
3。Natica，indeterminable。
4。Cyprinarostrata，d’Orbigny，“Pal。Franc。”(Neocomianformation)。
5。Rostellariaangulosa(？)，d’Orbigny，“Pal。del’Amer。Mer。”
6。Terebratula(？)。
SomeofthefragmentsofAmmoniteswereasthickasaman’sarm：theGryphaeaismuchthemostabundantshell。ThesefossilsM。d’OrbignyconsidersasbelongingtotheNeocomianstageoftheCretaceoussystem。Dr。
Meyen，whoascendedthevalleyoftheRioVolcan，abranchoftheYeso，foundanearlysimilar，butapparentlymorecalcareousformation，withmuchgypsum，andnodoubttheequivalentofthatheredescribed(“ReiseumErde“
etc。Th。1s。355。)：thebedswerevertical，andwereprolongeduptothelimitsofperpetualsnow;attheheightof9，000feetabovethesea，theyaboundedwithfossils，consisting，accordingtoVonBuch(“Descript。Phys。
desIlesCanaries“page471。)，of：——
1。Exogyra(Gryphaea)Couloni，absolutelyidenticalwithspecimensfromtheJuraandSouthofFrance。
2。Trigoniacostata，identicalwiththosefoundintheupperJurassicbedsatHildesheim。
3。Pectenstriatus，identicalwiththosefoundintheupperJurassicbedsatHildesheim。
4。Cucullaea，correspondinginformtoC。longirostris，sofrequentintheupperJurassicbedsofWestphalia。
5。AmmonitesresemblingA。biplex。
VonBuchconcludesthatthisformationisintermediatebetweenthelimestoneoftheJuraandthechalk，andthatitisanalogouswiththeuppermostJurassicbedsformingtheplainsofSwitzerland。HenceM。
D’OrbignyandVonBuch，underdifferentterms，comparethesefossilstothosefromthesamelatestageinthesecondaryformationsofEurope。
SomeofthefossilswhichIcollectedwerefoundagoodwaydownthewesternslopeofthemainridge，andhencemustoriginallyhavebeencoveredupbyagreatthicknessoftheblackshalyrock，independentlyofthenowdenuded，thick，overlyingmassesofredsandstone。Ineglectedatthetimetoestimatehowmanyhundredorratherthousandfeetthickthesuperincumbentstratamusthavebeen：andIwillnotnowattempttodoso。
This，however，wouldhavebeenahighlyinterestingpoint，asindicativeofagreatamountofsubsidence，ofwhichweshallhereafterfindinotherpartsoftheCordilleraanalogousevidenceduringthissameperiod。ThealtitudeofthePeuquenesRange，consideringitsnotgreatantiquity，isveryremarkable;manyofthefossilswereembeddedattheheightof13，210
feet，andthesamebedsareprolongeduptoatleastfromfourteentofifteenthousandfeetabovethelevelofthesea。
THEPORTILLOOREASTERNCHAIN。
ThevalleyofTenuyan，separatingthePeuquenesandPortillolines，is，asestimatedbyDr。Gilliesandmyself，abouttwentymilesinwidth;thelowestpart，wheretheroadcrossestheriver，being7，500feetabovethesea-level。ThepassonthePortillolineis14，365feethigh(1，100feethigherthanthatonthePeuquenes)，andtheneighbouringpinnaclesmust，I
conceive，risetonearly16，000feetabovethesea。TheriverdrainingtheintermediatevalleyofTenuyan，passesthroughthePortilloline。Toreturntooursection：——shortlyafterleavingthelowerbeds[P2]ofthegypseousformation，wecometograndmassesofacoarse，redconglomerate[V]，totallyunlikeanystratahithertoseenintheCordillera。Thisconglomerateisdistinctlystratified，someofthebedsbeingwelldefinedbythegreatersizeofthepebbles：thecementiscalcareousandsometimescrystalline，thoughthemassshowsnosignsofhavingbeenmetamorphosed。
Theincludedpebblesareeitherperfectlyoronlypartiallyrounded：theyconsistofpurplishsandstones，ofvariousporphyries，ofbrownishlimestone，ofblackcalcareous，compactshalepreciselylikethatinsituinthePeuquenesrange，andCONTAININGSOMEOFTHESAMEFOSSILSHELLS;alsoverymanypebblesofquartz，someofmicaceousschist，andnumerous，broken，roundedcrystalsofareddishorthiticorpotashfeldspar(asdeterminedbyProfessorMiller)，andthesefromtheirsizemusthavebeenderivedfromacoarse-grainedrock，probablygranite。Fromthisfeldsparbeingorthitic，andevenfromitsexternalappearance，Iventurepositivelytoaffirmthatithasnotbeenderivedfromtherocksofthewesternranges;but，ontheotherhand，itmaywellhavecome，togetherwiththequartzandmetamorphicschists，fromtheeasternorPortilloline，forthislinemainlyconsistsofcoarseorthiticgranite。Thepebblesofthefossiliferousslateandofthepurplesandstone，certainlyhavebeenderivedfromthePeuquenesorwesternranges。
TheroadcrossesthevalleyofTenuyaninanearlyeastandwestline，andforseveralmileswehaveonbothhandstheconglomerate，everywheredippingwestandformingseparategreatmountains。Thestrata，wherefirstmetwith，afterleavingthegypseousformation，areinclinedwestwardatanangleofonly20degrees，whichfurtheronincreasestoabout45degrees。
Thegypseousstrata，aswehaveseen，arealsoinclinedwestward：hence，whenlookingfromtheeasternsideofthevalleytowardsthePeuquenesrange，amostdeceptiveappearanceispresented，asifthenewerbedsofconglomeratedippeddirectlyunderthemucholderbedsofthegypseousformation。Inthemiddleofthevalley，aboldmountainofunstratifiedlilac-colouredporphyry(withcrystalsofhornblende)projects;andfurtheron，alittlesouthoftheroad，thereisanothermountain，withitsstratainclinedatasmallangleeastwards，whichinitsgeneralaspectandcolour，resemblestheporphyriticconglomerateformation，sorareonthissideofthePeuqueneslineandsograndlydevelopedthroughoutthewesternranges。
Theconglomerateisofgreatthickness：Idonotsupposethatthestrataformingtheseparatemountain-masses[V，V，V]haveeverbeenprolongedovereachother，butthatonemasshasbeenbrokenupbyseveral，distinct，parallel，uniclinallinesofelevation。Judgingthereforeofthethicknessoftheconglomerate，asseenintheseparatemountain-masses，Iestimateditatleastfromonethousandfivehundredtotwothousandfeet。Thelowerbedsrestconformablyonsomesingularlycoloured，softstrata[W]，whichI
couldnotreachtoexamine;andtheseagainrestconformablyonathickmassofmicaceous，thinlylaminated，siliceoussandstone[X]，associatedwithalittleblackclay-slate。Theselowerbedsaretraversedbyseveraldikesofdecomposingporphyry。Thelaminatedsandstoneisdirectlysuperimposedonthevastmassesofgranite[Y，Y]whichmainlycomposethePortillorange。Thelineofjunctionbetweenthislatterrock，whichisofabrightredcolour，andthewhitishsandstonewasbeautifullydistinct;
thesandstonebeingpenetratedbynumerous，great，tortuousdikesbranchingfromthegranite，andhavingbeenconvertedintoagranularquartzrock(singularlylikethatoftheFalklandIslands)，containingspecksofanochreypowder，andblackcrystallineatoms，apparentlyofimperfectmica。
Thequartzosestratainonespotwerefoldedintoaregulardome。
ThegranitewhichcomposesthemagnificentbarepinnaclesandthesteepwesternflankofthePortillochain，isofabrick-redcolour，coarselycrystallised，andcomposedoforthiticorpotashfeldspar，quartz，andimperfectmicainsmallquantity，sometimespassingintochlorite。Thesemineralsoccasionallyassumealaminarorfoliatedarrangement。Thefactofthefeldsparbeingorthiticinthisrange，isveryremarkable，consideringhowrare，orrather，asIbelieve，entirelyabsent，thismineralisthroughoutthewesternranges，inwhichsoda-feldspar，oratleastavarietycleavinglikealbite，issoextremelyabundant。Inonespotonthewesternflank，andontheeasternflanknearLosManantialesandnearthecrest，Inoticedsomegreatmassesofawhitishgranite，partsofitfine-
grained，andpartscontaininglargecrystalsoffeldspar;Ineglectedtocollectspecimens，soIdonotknowwhetherthisfeldsparisalsoorthitic，thoughIaminclinedtothinksofromitsgeneralappearance。Isawalsosomesyeniteandonemasswhichresembledandesite，butofwhichIlikewiseneglectedtocollectspecimens。Fromthemannerinwhichthewhitishgranitesformedseparatemountain-massesinthemidstofthebrick-redvariety，andfromonesuchmassnearthecrestbeingtraversedbynumerousveinsofflesh-colouredandgreenisheurite(intowhichIoccasionallyobservedthebrick-redgraniteinsensiblypassing)，Iconcludethatthewhitegranitesprobablybelongtoanolderformation，almostoverwhelmedandpenetratedbytheredgranite。
OnthecrestIsawalso，atashortdistance，somecolouredstratifiedbeds，apparentlylikethose[W]atthewesternbase，butwaspreventedexaminingthembyasnowstorm：Mr。Caldcleugh，however，collectedherespecimensofribbonedjasper，magnesianlimestone，andotherminerals。
(“Travels“etc。volume1page308。)Alittlewaydowntheeasternslopeafewfragmentsofquartzandmica-slatearemetwith;butthegreatformationofthislatterrock[Z]，whichcoversupmuchoftheeasternflankandbaseofthePortillorange，cannotbeconvenientlyexamineduntilmuchlowerdownataplacecalledMalPaso。Themica-schisthereconsistsofthicklayersofquartz，withinterveningfoliaoffinely-scalymica，oftenpassingintoasubstancelikeblackglossyclay-slate：inonespot，thelayersofthequartzhavingdisappeared，thewholemassbecameconvertedintoglossyclay-slate。Wherethefoliawerebestdefined，theywereinclinedatahighanglewestward，thatis，towardstherange。Thelineofjunctionbetweenthedarkmica-slateandthecoarseredgranitewasmostclearlydistinguishablefromavastdistance：thegranitesentmanysmallveinsintothemica-slate，andincludedsomeangularfragmentsofit。
Asthesandstoneonthewesternbasehasbeenconvertedbytheredgraniteintoagranularquartz-rock，sothisgreatformationofmica-schistmaypossiblyhavebeenmetamorphosedatthesametimeandbythesamemeans;
butIthinkitmoreprobable，consideringitsmoreperfectmetamorphiccharacteranditswell-pronouncedfoliation，thatitbelongstoananteriorepoch，connectedwiththewhitegranites：Iamthemoreinclinedtothisview，fromhavingfoundatthefootoftherangethemica-schistsurroundingahummock[Y2]，exclusivelycomposedofwhitegranite。NearLosArenales，themountainsonallsidesarecomposedofthemica-slate;andlookingbackwardsfromthispointuptothebaregiganticpeaksabove，theviewwaseminentlyinteresting。Thecoloursoftheredgraniteandtheblackmica-slatearesodistinct，thatwithabrightlighttheserockscouldbereadilydistinguishedevenfromthePampas，atalevelofatleast9，000feetbelow。Theredgranite，frombeingdividedbyparalleljoints，hasweatheredintosharppinnacles，onsomeofwhich，evenonsomeoftheloftiest，littlecapsofmica-schistcouldbeclearlyseen：hereandthereisolatedpatchesofthisrockadheredtothemountain-flanks，andtheseoftencorrespondedinheightandpositionontheoppositesidesoftheimmensevalleys。Lowerdowntheschistprevailedmoreandmore，withonlyafewquitesmallpointsofgraniteprojectingthrough。LookingattheentireeasternfaceofthePortillorange，theredcolourfarexceedsinareatheblack;yetitwasscarcelypossibletodoubtthatthegranitehadoncebeenalmostwhollyencasedbythemica-schist。
AtLosArenales，lowdownontheeasternflank，themica-slateistraversedbyseveralcloselyadjoining，broaddikes，paralleltoeachotherandtothefoliationoftheschist。Thedikesareformedofthreedifferentvarietiesofrock，ofwhichapalebrownfeldspathicporphyrywithgrainsofquartzwasmuchthemostabundant。Thesedikeswiththeirgranulesofquartz，aswellasthemica-schistitself，strikinglyresembletherocksoftheChonosArchipelago。Ataheightofabouttwelvehundredfeetabovethedikes，andperhapsconnectedwiththem，thereisarangeofcliffsformedofsuccessivelava-streams[AA]，betweenthreeandfourhundredfeetinthickness，andinplacesfinelycolumnar。Thelavaconsistsofdark-
greyish，harshrocks，intermediateincharacterbetweentrachyteandbasalt，containingglassyfeldspar，olivine，andalittlemica，andsometimesamygdaloidalwithzeolite：thebasisiseitherquitecompact，orcrenulatedwithair-vesiclesarrangedinlaminae。Thestreamsareseparatedfromeachotherbybedsoffragmentarybrownscoriae，firmlycementedtogether，andincludingafewwell-roundedpebblesoflava。Fromtheirgeneralappearance，Isuspectthattheselava-streamsflowedatanancientperiodunderthepressureofthesea，whentheAtlanticcoveredthePampasandwashedtheeasternfootoftheCordillera。(Thisconclusionmight，perhaps，evenhavebeenanticipated，fromthegeneralrarityofvolcanicaction，exceptneartheseaorlargebodiesofwater。Conformablywiththisrule，atthepresentday，therearenoactivevolcanoesonthiseasternsideoftheCordillera;noraresevereearthquakesexperiencedhere。)Ontheoppositeandnorthernsideofthevalleythereisanotherlineoflava-
cliffsatacorrespondingheight;thevalleybetweenbeingofconsiderablebreadth，andasnearlyasIcouldestimate1，500feetindepth。Thisfieldoflavaisconfinedonbothsidesbythemountainsofmica-schist，andslopesdownrapidlybutirregularlytotheedgeofthePampas，where，havingathicknessofabouttwohundredfeet，itterminatesagainstalittlerangeofclaystoneporphyry。Thevalleyinthislowerpartexpandsintoabay-like，gentleslope，borderedbythecliffsoflava，whichmustcertainlyoncehaveextendedacrossthiswideexpanse。TheinclinationofthestreamsfromLosArenalestothemouthofthevalleyissogreat，thatatthetime(thoughignorantofM。EliedeBeaumont’sresearchesontheextremelysmallslopeoverwhichlavacanflow，andyetretainacompactstructureandconsiderablethickness)Iconcludedthattheymustsubsequentlytotheirflowinghavebeenupheavedandtiltedfromthemountains;ofthisconclusionIcannowentertainnotthesmallestdoubt。
Atthemouthofthevalley，withinthecliffsoftheabovelava-field，thereareremnants，intheformofseparatesmallhillocksandoflinesoflowcliffs，ofaconsiderabledepositofcompactwhitetuff(quarriedforfiltering-stones)，composedofbrokenpumice，volcaniccrystals，scalesofmica，andfragmentsoflava。Thismasshassufferedmuchdenudation;andthehardmica-schisthasbeendeeplyworn，sincetheperiodofitsdeposition;andthisperiodmusthavebeensubsequenttothedenudationofthebasalticlava-streams，asattestedbytheirencirclingcliffsstandingatahigherlevel。Atthepresentday，undertheexistingaridclimate，agesmightrollpastwithoutasquareyardofrockofanykindbeingdenuded，exceptperhapsintherarelymoisteneddrainage-channelofthevalley。Mustwethenlookbacktothatancientperiod，whenthewavesoftheseabeatagainsttheeasternfootoftheCordillera，forapowersufficienttodenudeextensively，thoughsuperficially，thistufaceousdeposit，softalthoughitbe？
Thereremainsonlytomentionsomelittlewater-wornhillocks[BB]，afewhundredfeetinheight，andmeremole-hillscomparedwiththegiganticmountainsbehindthem，whichriseoutofthesloping，shingle-coveredmarginofthePampas。Thefirstlittlerangeiscomposedofabrecciatedpurpleporphyriticclaystone，withobscurelymarkedstratadippingat70
degreestotheS。W。;theotherrangesconsistof——apale-colouredfeldspathicporphyry，——apurpleclaystoneporphyrywithgrainsofquartz，——
andarockalmostexclusivelycomposedofbrick-redcrystalsoffeldspar。
TheseoutermostsmalllinesofelevationextendinaN。W。byW。andS。E。byS。direction。
CONCLUDINGREMARKSONTHEPORTILLORANGE。
WhenonthePampasandlookingsouthward，andwhilsttravellingnorthward，Icouldseeforverymanyleaguestheredgraniteanddarkmica-schistformingthecrestandeasternflankofthePortilloline。Thisgreatrange，accordingtoDr。Gillies，canbetracedwithlittleinterruptionfor140
milessouthwardtotheR。Diamante，whereituniteswiththewesternranges：northward，accordingtothissameauthor，itterminateswheretheR。Mendozadebouchesfromthemountains;butalittlefurthernorthintheeasternpartoftheCumbresection，thereare，asweshallhereaftersee，somemountain-massesofabrick-redporphyry，thelastinjectedamidstmanyotherporphyries，andhavingsocloseananalogywiththecoarseredgraniteofthePortilloline，thatIamtemptedtobelievethattheybelongtothesameaxisofinjection;ifso，thePortillolineisatleast200
milesinlength。Itsheight，eveninthelowestgapintheroad，is14，365
feet，andsomeofthepinnaclesapparentlyattainanelevationofabout16，000feetabovethesea。Thegeologicalhistoryofthisgrandchainappearstomeeminentlyinteresting。Wemaysafelyconclude，thatataformerperiodthevalleyofTenuyanexistedasanarmofthesea，abouttwenty-milesinwidth，borderedononehandbyaridgeorchainofisletsoftheblackcalcareousshalesandpurplesandstonesofthegypseousformation;andontheotherhand，byaridgeorchainofisletscomposedofmica-slate，whitegranite，andperhapstoapartialextentofredgranite。
Thesetwochains，whilstthusborderingtheoldsea-channel，musthavebeenexposedforavastlapseoftimetoalluvialandlittoralaction，duringwhichtherockswereshattered，thefragmentsrounded，andthestrataofconglomerateaccumulatedtoathicknessofatleastfifteenhundredortwothousandfeet。Theredorthiticgranitenowforms，aswehaveseen，themainpartofthePortillochain：itisinjectedindikesnotonlyintothemica-schistandwhitegranites，butintothelaminatedsandstone，whichithasmetamorphosed，andwhichithasthrownoff，togetherwiththeconformablyoverlyingcolouredbedsandstratifiedconglomerate，atanangleofforty-fivedegrees。Tohavethrownoffsovastapileofstrataatthisangle，isaproofthatthemainpartoftheredgranite(whetherornotportions，asperhapsisprobable，previouslyexisted)wasinjectedinaliquifiedstateaftertheaccumulationbothofthelaminatedsandstoneandoftheconglomerate;thisconglomerate，weknow，wasaccumulated，notonlyafterthedepositionofthefossiliferousstrataofthePeuquenesline，butaftertheirelevationandlong-continueddenudation：andthesefossiliferousstratabelongtotheearlypartoftheCretaceoussystem。
Late，therefore，inageologicalsense，asmustbetheageofthemainpartoftheredgranite，Icanconceivenothingmoreimpressivethantheeasternviewofthisgreatrange，asforcingthemindtograpplewiththeideaofthethousandsofthousandsofyearsrequisiteforthedenudationofthestratawhichoriginallyencasedit，——forthatthefluidifiedgranitewasonceencased，itsmineralogicalcompositionandstructure，andtheboldconicalshapeofthemountain-masses，yieldsufficientevidence。Oftheencasingstrataweseethelastvestigesinthecolouredbedsonthecrest，inthelittlecapsofmica-schistonsomeoftheloftiestpinnacles，andintheisolatedpatchesofthissamerockatcorrespondingheightsonthenowbareandsteepflanks。
Thelava-streamsattheeasternfootofthePortilloareinteresting，notsomuchfromthegreatdenudationwhichtheyhavesufferedatacomparativelylateperiodasfromtheevidencetheyaffordbytheirinclinationtakenconjointlywiththeirthicknessandcompactness，thatafterthegreatrangehadassumeditspresentgeneraloutline，itcontinuedtoriseasanaxisofelevation。TheplainsextendingfromthebaseoftheCordilleratotheAtlanticshowthatthecontinenthasbeenupraisedinmasstoaheightof3，500feet，andprobablytoamuchgreaterheight，forthesmoothshingle-coveredmarginofthePampasisprolongedinagentleunbrokenslopefarupmanyofthegreatvalleys。NorletitbeassumedthatthePeuquenesandPortillorangeshaveundergoneonlymovementsofelevation;forweshallhereaftersee，thatthebottomoftheseasubsidedseveralthousandfeetduringthedepositionofstrata，occupyingthesamerelativeplaceintheCordillera，withthoseofthePeuquenesridge;
moreover，weshallseefromtheunequivocalevidenceofburieduprighttrees，thatatasomewhatlaterperiod，duringtheformationoftheUspallatachain，whichcorrespondsgeographicallywiththatofthePortillo，therewasanothersubsidenceofmanythousandfeet：here，indeed，inthevalleyofTenuyan，theaccumulationofthecoarsestratifiedconglomeratetoathicknessoffifteenhundredortwothousandfeet，offersstrongpresumptiveevidenceofsubsidence;forallexistinganalogiesleadtothebeliefthatlargepebblescanbetransportedonlyinshallowwater，liabletobeaffectedbycurrentsandmovementsofundulation——andifso，theshallowbedoftheseaonwhichthepebbleswerefirstdepositedmustnecessarilyhavesunktoallowoftheaccumulationofthesuperincumbentstrata。Whatahistoryofchangesoflevel，andofwearandtear，allsincetheageofthelattersecondaryformationsofEurope，doesthestructureofthisonegreatmountain-chainreveal！
PASSAGEOFTHEANDESBYTHECUMBREORUSPALLATAPASS。
ThisPasscrossestheAndesaboutsixtymilesnorthofthatjustdescribed：
thesectiongiveninPlate1，Section1/2，isonthesamescaleasbefore，namely，atone-thirdofaninchtoamileindistance，andoneinchtoamile(or6，000feet)inheight。Likethelastsection，itisameresketch，andcannotpretendtoaccuracy，thoughmadeunderfavourablecircumstances。
Wewillcommenceasbefore，withthewesternhalf，ofwhichthemainrangebearsthenameoftheCumbre(thatistheRidge)，andcorrespondstothePeuqueneslineintheformersection;asdoestheUspallatarange，thoughonamuchsmallerscale，tothatofthePortillo。NearthepointwheretheriverAconcaguadebouchesonthebasinplainofthesamename，ataheightofabouttwothousandthreehundredfeetabovethesea，wemeetwiththeusualpurpleandgreenishporphyriticclaystoneconglomerate。Bedsofthisnature，alternatingwithnumerouscompactandamygdaloidalporphyries，whichhaveflowedassubmarinelavas，andassociatedwithgreatmountain-
massesofvarious，injected，non-stratifiedporphyries，areprolongedthewholedistanceuptotheCumbreorcentralridge。Oneofthecommoneststratifiedporphyriesisofagreencolour，highlyamygdaloidalwiththevariousmineralsdescribedinthepreliminarydiscussion，andincludingfinetabularcrystalsofalbite。Themountain-rangenorth(oftenwithalittlewesting)andsouth。Thestratification，whereverIcouldclearlydistinguishit，wasinclinedwestwardortowardsthePacific，and，exceptneartheCumbre，seldomatanglesabove25degrees。Onlyatonespotonthiswesternside，onaloftypinnaclenotfarfromtheCumbre，Isawstrataapparentlybelongingtothegypseousformation，andconformablycappingapileofstratifiedporphyries。Hence，bothincompositionandinstratification，thestructureofthemountainsonthiswesternsideofthedivortiumaquarum，isfarmoresimplethaninthecorrespondingpartofthePeuquenessection。Intheporphyriticclaystoneconglomerate，themechanicalstructureandtheplanesofstratificationhavegenerallybeenmuchobscuredandevenquiteobliteratedtowardsthebaseoftheseries，whilstintheupperparts，nearthesummitsofthemountains，botharedistinctlydisplayed。Intheseupperportionstheporphyriesaregenerallylightercoloured。Inthreeplaces[X，Y，Z]massesofandesiteareexposed：
at[Y]，thisrockcontainedsomequartz，butthegreaterpartconsistedofandesiticporphyry，withonlyafewwell-developedcrystalsofalbite，andformingagreatwhitemass，havingtheexternalaspectofgranite，cappedbymuchdarkunstratifiedporphyry。Inmanypartsofthemountains，therearedikesofagreencolour，andotherwhiteones，whichlatterprobablyspringfromunderlyingmassesofandesite。
TheCumbre，wheretheroadcrossesit，is，accordingtoMr。Pentland，12，454feetabovethesea;andtheneighbouringpeaks，composedofdarkpurpleandwhitishporphyries，someobscurelystratifiedwithawesterlydip，andotherswithoutatraceofstratification，mustexceed13，000feetinheight。DescendingtheeasternslopeoftheCumbre，thestructurebecomesverycomplicated，andgenerallydiffersonthetwosidesoftheeastandwestlineofroadandsection。Firstwecometoagreatmass[A]
ofnearlyvertical，singularlycontortedstrata，composedofhighlycompactredsandstones，andofoftencalcareousconglomerates，andpenetratedbygreen，yellow，andreddishdikes;butIshallpresentlyhaveanopportunityofdescribinginsomedetailananalogouspileofstrata。Theseverticalbedsareabruptlysucceededbyothers[B]，ofapparentlynearlythesamenaturebutmoremetamorphosed，alternatingwithporphyriesandlimestones;
thesedipforashortspacewestward，buttherehasbeenhereanextraordinarydislocation，which，onthenorthsideoftheroad，appearstohavedeterminedtheexcavationofthenorthandsouthvalleyoftheR。delasCuevas。Onthisnorthernsideoftheroad，thestrata[B]areprolongedtilltheycomeinclosecontactwithajaggedloftymountain[D]ofdark-
coloured，unstratified，intrusiveporphyry，wherethebedshavebeenmorehighlyinclinedandstillmoremetamorphosed。Thismountainofporphyryseemstoformashortaxisofelevation，forsouthoftheroadinitslinethereisahill[C]ofporphyriticconglomeratewithabsolutelyverticalstrata。
Wenowcometothegypseousformation：Iwillfirstdescribethestructureoftheseveralmountains，andthengiveinonesectionadetailedaccountofthenatureoftherocks。Onthenorthsideoftheroad，whichhererunsinaneastandwestvalley，themountainofporphyry[D]issucceededbyahill[E]formedoftheuppergypseousstratatilted，atanangleofbetween70and80degreestothewest，byauniclinalaxisofelevationwhichdoesnotrunparalleltotheotherneighbouringranges，andwhichisofshortlength;foronthesouthsideofthevalleyitsprolongationismarkedonlybyasmallflexureinapileofstratainclinedbyaquiteseparateaxis。A
littlefurtheronthenorthandsouthvalleyofHorconesentersatrightanglesourlineofsection;itswesternsideisboundedbyahillofgypseousstrata[F]dippingwestwardatabout45degrees，anditseasternsidebyamountainofsimilarstrata[G]inclinedwestwardat70degrees，andsuperimposedbyanobliquefaultonanothermassofthesamestrata[H]，alsoinclinedwestward，butatanangleofabout30degrees：thecomplicatedrelationofthesethreemasses[F，G，H]isexplainedbythestructureofagreatmountain-rangelyingsomewaytothenorth，inwhicharegularanticlinalaxis(representedinthesectionbydottedlines)isseen，withthestrataonitseasternsideagainbendingupandformingadistinctuniclinalaxis，ofwhichthebedsmarked[H]formthelowerpart。
Thisgreatuniclinallineisintersected，nearthePuentedelInca，bythevalleyalongwhichtheroadruns，andthestratacomposingitwillbeimmediatelydescribed。Onthesouthsideoftheroad，inthespacecorrespondingwiththemountains[E，F，andG]，thestrataeverywheredipwestwardgenerallyatanangleof30degrees，occasionallymountingupto45degrees，butnotinanunbrokenline，forthereareseveralverticalfaults，formingseparateuniclinalmasses，alldippinginthesamedirection，——aformofelevationcommonintheCordillera。Wethusseethatwithinanarrowspace，thegypseousstratahavebeenupheavedandcrushedtogetherbyagreatuniclinal，anticlinal，andonelesseruniclinalline[E]ofelevation;andthatbetweenthesethreelinesandtheCumbre，inthesandstones，conglomeratesandporphyriticformation，therehavebeenatleasttwoorthreeothergreatelevatoryaxes。
Theuniclinalaxis[I]intersectednearthePuentedelInca(ofwhichthestrataat[H]formapart)rangesN。byW。andS。byE。formingachainofmountains，apparentlylittleinferiorinheighttotheCumbre：thestrata，aswehaveseen，dipatanaverageangleof30degreestothewest。(Atthisplace，therearesomehotandcoldsprings，thewarmesthavingatemperature，accordingtoLieutenantBrand“Travels，“page240，of91
degrees;theyemitmuchgas。AccordingtoMr。Brande，oftheRoyalInstitution，tencubicalinchescontainforty-fivegrainsofsolidmatter，consistingchieflyofsalt，gypsum，carbonateoflime，andoxideofiron。
Thewaterischargedwithcarbonicacidandsulphurettedhydrogen。Thesespringsdepositmuchtufaintheformofsphericalballs。Theyburstforth，asdothoseofCauquenes，andprobablythoseofVillaVicencio，onalineofelevation。)Theflanksofthemountainsareherequitebareandsteep，affordinganexcellentsection;sothatIwasabletoinspectthestratatoathicknessofabout4，000feet，andcouldclearlydistinguishtheirgeneralnaturefor1，000feethigher，makingatotalthicknessof5，000
feet，towhichmustbeaddedabout1，000feetoftheinferiorstrataseenalittlelowerdownthevalley，Iwilldescribethisonesectionindetail，beginningatthebottom。
1st。Thelowestmassisthealteredclay-slatedescribedinthepreliminarydiscussion，andwhichinthislineofsectionwasherefirstmetwith。
Lowerdownthevalley，attheR。delasVacas，Ihadabetteropportunityofexaminingit;itisthereinsomepartswellcharacterised，havingadistinct，nearlyvertical，tortuouscleavage，rangingN。W。andS。E。andintersectedbyquartzveins：inmostparts，however，itiscrystallineandfeldspathic，andpassesintoatruegreenstoneoftenincludinggrainsofquartz。Theclay-slate，initsupperhalf，isfrequentlybrecciated，theembeddedangularfragmentsbeingofnearlythesamenaturewiththepaste。
2nd。Severalstrataofpurplishporphyriticconglomerate，ofnoverygreatthickness，restconformablyuponthefeldspathicslate。Athickbedoffine，purple，claystoneporphyry，obscurelybrecciated(butnotofmetamorphosedsedimentaryorigin)，andcappedbyporphyriticconglomerate，wasthelowestbedactuallyexaminedinthissectionatthePuentedelInca。
3rd。Astratum，eightyfeetthick，ofhardandverycompactimpurewhitishlimestone，weatheringbrightred，withincludedlayersbrecciatedandre-
cemented。Obscuremarksofshellaredistinguishableinit。
4th。Ared，quartzose，fine-grainedconglomerate，withgrainsofquartz，andwithpatchesofwhiteearthyfeldspar，apparentlyduetosomeprocessofconcretionarycrystallineaction;thisbedismorecompactandmetamorphosedthananyoftheoverlyingconglomerates。
5th。Awhitishchertylimestone，withnodulesofbluishargillaceouslimestone。
6th。Awhiteconglomerate，withmanyparticlesofquartz，almostblendingintothepaste。
7th。Highlysiliceous，fine-grainedwhitesandstone。
8thand9th。Redandwhitebedsnotexamined。
10th。Yellow，fine-grained，thinlystratified，magnesian(judgingfromitsslowdissolutioninacids)limestone：itincludessomewhitequartzpebbles，andlittlecavities，linedwithcalcareousspar，someretainingtheformofshells。
11th。Abedbetweentwentyandthirtyfeetthick，quiteconformablewiththeunderlyingones，composedofahardbasis，tingedlilac-greyporphyriticwithNUMEROUScrystalsofwhitishfeldspar，withblackmicaandlittlespotsofsoftferruginousmatter：evidentlyasubmarinelava。
12th。Yellowmagnesianlimestone，asbefore，part-stainedpurple。
13th。Amostsingularrock;basispurplishgrey，obscurelycrystalline，easilyfusibleintoadarkgreenglass，nothard，thicklyspeckledwithcrystalsmoreorlessperfectofwhitecarbonateoflime，ofredhydrousoxideofiron，ofawhiteandtransparentminerallikeanalcime，andofagreenopaqueminerallikesoap-stone;thebasisismoreoveramygdaloidalwithmanysphericalballsofwhitecrystallisedcarbonateoflime，ofwhichsomearecoatedwiththeredoxideofiron。Ihavenodoubt，fromtheexaminationofasuperincumbentstratum(19)，thatthisisasubmarinelava;thoughinNorthernChile，someofthemetamorphosedsedimentarybedsarealmostascrystalline，andofasvariedcomposition。
14th。Redsandstone，passingintheupperpartintoacoarse，hard，redconglomerate，300feetthick，havingacalcareouscement，andincludinggrainsofquartzandbrokencrystalsoffeldspar;basisinfusible;thepebblesconsistofdullpurplishporphyries，withsomeofquartz，fromthesizeofanuttoaman’shead。ThisisthecoarsestconglomerateinthispartoftheCordillera：inthemiddletherewasawhitelayernotexamined。
15th。Grandthickbed，ofaveryhard，yellowish-whiterock，withacrystallinefeldspathicbase，includinglargecrystalsofwhitefeldspar，manylittlecavitiesmostlyfullofsoftferruginousmatter，andnumeroushexagonalplatesofblackmica。Theupperpartofthisgreatbedisslightlycellular;thelowerpartcompact：thethicknessvariedalittleindifferentparts。Manifestlyasubmarinelava;andisalliedtobed11。
16thand17th。Dullpurplish，calcareous，fine-grained，compactsandstones，whichpassintocoarsewhiteconglomerateswithnumerousparticlesofquartz。
18th。Severalalternationsofredconglomerate，purplishsandstone，andsubmarinelava，likethatsingularrockformingbed13。
19th。Averyheavy，compact，greenish-blackstone，withafine-grainedobviouslycrystallinebasis，containingafewspecksofwhitecalcareousspar，manyspecksofthecrystallisedhydrousredoxideofiron，andsomespecksofagreenmineral;thereareveinsandnestsfilledwithepidote：
certainlyasubmarinelava。
20th。Manythinstrataofcompact，fine-grained，palepurplesandstone。
21st。Gypsuminanearlypurestate，aboutthreehundredfeetinthickness：
thisbed，initsconcretionsofanhydriteandlayersofsmallblackishcrystalsofcarbonateoflime，exactlyresemblesthegreatgypseousbedsinthePeuquenesrange。
22nd。Palepurpleandreddishsandstone，asinbed20：aboutthreehundredfeetinthickness。
23rd。Athickmasscomposedoflayers，oftenasthinaspaperandconvoluted，ofpuregypsumwithothersveryimpure，ofapurplishcolour。
24th。Puregypsum，thickmass。
25th。Redsandstones，ofgreatthickness。
26th。Puregypsum，ofgreatthickness。
27th。Alternatinglayersofpureandimpuregypsum，ofgreatthickness。
Iwasnotabletoascendtothesefewlastgreatstrata，whichcomposetheneighbouringloftiestpinnacles。Thethickness，fromthelowesttotheuppermostbedofgypsum，cannotbelessthan2，000feet：thebedsbeneathI
estimatedat3，000feet，andthisdoesnotincludeeitherthelowerpartsoftheporphyriticconglomerate，orthealteredclay-slate;Iconceivethetotalthicknessmustbeaboutsixthousandfeet。Idistinctlyobservedthatnotonlythegypsum，butthealternatingsandstonesandconglomerateswerelens-shaped，andrepeatedlythinnedoutandreplacedeachother：thusinthedistanceofaboutamile，abed300feetthickofsandstonebetweentwobedsofgypsum，thinnedouttonothinganddisappeared。Thelowerpartofthissectiondiffersremarkably，——inthemuchgreaterdiversityofitsmineralogicalcomposition，——intheabundanceofcalcareousmatter，——inthegreatercoarsenessofsomeoftheconglomerates，——andinthenumerousparticlesandwell-roundedpebbles，sometimesoflargesize，ofquartz，——
fromanyothersectionhithertodescribedinChile。Fromthesepeculiaritiesandfromthelens-formofthestrata，itisprobablethatthisgreatpileofstratawasaccumulatedonashallowandveryunevenbottom，nearsomepre-existinglandformedofvariousporphyriesandquartz-rock。Theformationofporphyriticclaystoneconglomeratedoesnotinthissectionattainnearlyitsordinarythickness;thismaybePARTLY
attributedtothemetamorphicactionhavingbeenheremuchlessenergeticthanusual，thoughthelowerbedshavebeenaffectedtoacertaindegree。
IfithadbeenasenergeticasinmostotherpartsofChile，manyofthebedsofsandstoneandconglomerate，containingroundedmassesofporphyry，woulddoubtlesshavebeenconvertedintoporphyriticconglomerate;andthesewouldhavealternatedwith，andevenblendedinto，crystallineandporphyriticstratawithoutatraceofmechanicalstructure，——namely，intothosewhich，inthepresentstateofthesection，weseeareunquestionablysubmarinelavas。
Thebedsofgypsum，togetherwiththeredalternatingsandstonesandconglomerates，presentsoperfectandcuriousaresemblancewiththoseseeninourformersectioninthebasin-valleyofYeso，thatIcannotdoubttheidentityofthetwoformations：Imayadd，thatalittlewestwardoftheP。
delInca，amassofgypsumpassedintoafine-grained，hard，brownsandstone，whichcontainedsomelayersofblack，calcareous，compact，shalyrock，preciselylikethatseeninsuchvastmassesonthePeuquenesrange。
NearthePuentedelInca，numerousfragmentsoflimestone，containingsomefossilremains，werescatteredontheground：thesefragmentssoperfectlyresemblethelimestoneofbedNo。3，inwhichIsawimpressionsofshells，thatIhavenodoubttheyhavefallenfromit。TheyellowmagnesianlimestoneofbedNo。10，whichalsoincludestracesofshells，hasadifferentappearance。Thesefossils(asnamedbyM。d’Orbigny)consistof：——
Gryphaea，neartoG。Couloni(Neocomianformation)。
Arca，perhapsA。Gabrielis，d’Orbigny，“Pal。Franc。”(Neocomianformation)。
Mr。Pentlandmadeacollectionofshellsfromthissamespot，andVonBuchconsidersthemasconsistingof：——
Trigonia，resemblinginformT。costata。
Pholadomya，likeonefoundbyM。DufresnoynearAlencon。
Isocardiexcentrica，Voltz。identicalwiththatfromtheJura。
(“DescriptionPhys。desIlesCan。”page472。)
Twooftheseshells，namely，theGryphaeaandTrigonia，appeartobeidenticalwithspeciescollectedbyMeyenandmyselfonthePeuquenesrange;andintheopinionofVonBuchandM。d’Orbigny，thetwoformationsbelongtothesameage。Imusthereadd，thatProfessorE。Forbes，whohasexaminedmyspecimensfromthisplaceandfromthePeuquenesrange，haslikewiseastrongimpressionthattheyindicatetheCretaceousperiod，andprobablyanearlyepochinit：sothatallthepalaeontologistswhohaveseenthesefossilsnearlycoincideinopinionregardingtheirage。Thelimestone，however，withthesefossilshereliesattheverybaseoftheformation，justabovetheporphyriticconglomerate，andcertainlyseveralthousandfeetlowerintheseries，thantheequivalent，fossiliferous，black，shalyrockshighuponthePeuquenesrange。
Itiswellworthyofremarkthattheseshells，oratleastthoseofwhichI
sawimpressionsinthelimestone(bedNo。3)，musthavebeencoveredup，ontheLEASTcomputation，by4，000feetofstrata：nowweknowfromProfessorE。Forbes’sresearches，thattheseaatgreaterdepthsthan600feetbecomesexceedinglybarrenoforganicbeings，——aresultquiteinaccordancewithwhatlittleIhaveseenofdeep-seasoundings。Hence，afterthislimestonewithitsshellswasdeposited，thebottomoftheseawherethemainlineoftheCordilleranowstands，musthavesubsidedsomethousandfeettoallowofthedepositionofthesuperincumbentsubmarinestrata。
Withoutsupposingamovementofthiskind，itwould，moreover，beimpossibletounderstandtheaccumulationoftheseverallowerstrataofCOARSE，well-roundedconglomerates，whichitisscarcelypossibletobelievewerespreadoutinprofoundlydeepwater，andwhich，especiallythosecontainingpebblesofquartz，couldhardlyhavebeenroundedinsubmarinecratersandafterwardsejectedfromthem，asIbelievetohavebeenthecasewithmuchoftheporphyriticconglomerateformation。Imayaddthat，inProfessorForbes’sopinion，theabove-enumeratedspeciesofmolluscaprobablydidnotliveatamuchgreaterdepththantwentyfathoms，thatisonly120feet。
Toreturntooursectiondownthevalley;standingonthegreatN。byW。
andS。byE。uniclinalaxisofthePuentedelInca，ofwhichasectionhasjustbeengiven，andlookingnorth-east，greatertabularmassesofgypseousformation(KK)couldbeseeninthedistance，veryslightlyinclinedtowardstheeast。Lowerdownthevalley，themountainsarealmostexclusivelycomposedofporphyries，manyofthemofintrusiveoriginandnon-stratified，othersstratified，butwiththestratificationseldomdistinguishableexceptintheupperparts。Disregardinglocaldisturbances，thebedsareeitherhorizontalorinclinedatasmallangleeastwards：
hence，whenstandingontheplainofUspallataandlookingtothewestorbackwards，theCordilleraappearcomposedofhuge，square，nearlyhorizontal，tabularmasses：sowideaspace，withsuchloftymountainssoequablyelevated，israrelymetwithwithintheCordillera。Inthislineofsection，theintervalbetweenthePuentedelIncaandtheneighbourhoodoftheCumbre，includesallthechiefaxesofdislocation。
Thealteredclay-slateformation，alreadydescribed，isseeninseveralpartsofthevalleyasfardownasLasVacas，underlyingtheporphyriticconglomerate。AttheCasadePujios[L]，thereisahummockof(andesitic？)
granite;andthestratificationofthesurroundingmountainsherechangesfromW。byS。toS。W。Again，neartheR。Vacasthereisalargerformationof(andesitic？)granite[M]，whichsendsameshworkofveinsintothesuperincumbentclay-slate，andwhichlocallythrowsoffthestrata，ononesidetoN。W。andontheothertoS。E。butnotatahighangle：atthejunction，theclay-slateisalteredintofine-grainedgreenstone。Thisgraniticaxisisintersectedbyagreendike，whichImention，becauseIdonotrememberhavingelsewhereseendikesinthislowestandlatestintrusiverock。FromtheR。VacastotheplainofUspallata，thevalleyrunsN。E。sothatIhavehadtocontractmysection;itrunsexclusivelythroughporphyriticrocks。AsfarasthePassofJaula，theclaystoneconglomerateformation，inmostpartshighlyporphyritic，andcrossedbynumerousdikesofgreenstoneporphyry，attainsagreatthickness：thereisalsomuchintrusiveporphyry。FromtheJaulatotheplain，thestratificationhasbeeninmostplacesobliterated，exceptnearthetopsofsomeofthemountains;andthemetamorphicactionhasbeenextremelygreat。
Inthisspace，thenumberandbulkoftheintrusivemassesofdifferentlycolouredporphyries，injectedoneintoanotherandintersectedbydikes，istrulyextraordinary。Isawonemountainofwhitishporphyry，fromwhichtwohugedikes，thinningout，branchedDOWNWARDSintoanadjoiningblackishporphyry。Anotherhillofwhiteporphyry，whichhadburstthroughdark-
colouredstrata，wasitselfinjectedbyapurple，brecciated，andrecementedporphyry，bothbeingcrossedbyagreendike，andbothhavingbeenupheavedandinjectedbyagraniticdome。Onebrick-redporphyry，whichabovetheJaulaformsanisolatedmassinthemidstoftheporphyriticconglomerateformation，andlowerdownthevalleyamagnificentgroupofpeakedmountains，differsremarkablyfromalltheotherporphyries。Itconsistsofaredfeldspathicbase，includingsomeratherlargecrystalsofredfeldspar，numerouslargeangulargrainsofquartz，andlittlebitsofasoftgreenmineralansweringinmostofitscharacterstosoapstone。Thecrystalsofredfeldsparresembleinexternalappearancethoseoforthite，though，frombeingpartiallydecomposed，Iwasunabletomeasurethem;andtheycertainlyarequiteunlikethevariety，soabundantlymetwithinalmostalltheotherrocksofthislineofsection，andwhich，whereverItriedit，cleavedlikealbite。Thisbrick-redporphyryappearstohaveburstthroughalltheotherporphyries，andnumerousreddikestraversingtheneighbouringmountainshaveproceededfromit：insomefewplaces，however，itwasintersectedbywhitedikes。
>Fromthisposteriorityofintrusiveorigin，——fromtheclosegeneralresemblancebetweenthisredporphyryandtheredgraniteofthePortilloline，theonlydifferencebeingthatthefeldsparhereislessperfectlygranular，andthatsoapstonereplacesthemica，whichisthereimperfectandpassesintochlorite，——andfromthePortillolinealittlesouthwardofthispointappearingtoblend(accordingtoDr。Gillies)intothewesternranges，——Iamstronglyurgedtobelieve(asformerlyremarked)thatthegrandmountain-massescomposedofthisbrick-redporphyrybelongtothesameaxisofinjectionwiththegraniteofthePortilloline;ifso，theinjectionofthisporphyryprobablytookplace，aslongsubsequentlytotheseveralaxesofelevationinthegypseousformationneartheCumbre，astheinjectionofthePortillogranitehasbeenshowntohavebeensubsequenttotheelevationofthegypseousstratacomposingthePeuquenesrange;andthisinterval，wehaveseen，musthavebeenaverylongone。
ThePlainofUspallatahasbeenbrieflydescribedinChapter3;itresemblesthebasin-plainsofChile;itistenorfifteenmileswide，andissaidtoextendfor180milesnorthward;itssurfaceisnearlysixthousandfeetabovethesea;itiscomposed，toathicknessofsomehundredfeetoflooselyaggregated，stratifiedshingle，whichisprolongedwithagentlyslopingsurfaceupthevalleysinthemountainsonbothsides。Onesectioninthisplain[Z]isinteresting，fromtheunusualcircumstanceofalternatinglayersofalmostlooseredandwhitesandwithlinesofpebbles(fromthesizeofanuttothatofanapple)，andbedsofgravel，beinginclinedatanangleof45degrees，andinsomespotsevenatahigherangle。(IfindthatMr。SmithofJordanHillhasdescribed(“EdinburghNewPhilosophicalJournal“volume25page392)bedsofsandandgravel，nearEdinburgh，tiltedatanangleof60degrees，anddislocatedbyminiaturefaults。)Thesebedsaredislocatedbysmallfaults：andarecappedbyathickmassofhorizontallystratifiedgravel，evidentlyofsubaqueousorigin。Havingbeenaccustomedtoobservetheirregularitiesofbedsaccumulatedundercurrents，Ifeelsurethattheinclinationherehasnotbeenthusproduced。Thepebblesconsistchieflyofthebrick-redporphyryjustdescribedandofwhitegranite，bothprobablyderivedfromtherangestothewest，andofalteredclay-slateandofcertainporphyries，apparentlybelongingtotherocksoftheUspallatachain。ThisplaincorrespondsgeographicallywiththevalleyofTenuyanbetweenthePortilloandPeuquenesranges;butinthatvalleytheshingle，whichlikewisehasbeenderivedbothfromtheeasternandwesternranges，hasbeencementedintoahardconglomerate，andhasbeenthroughouttiltedataconsiderableinclination;thegravelthereapparentlyattainsamuchgreaterthickness，andisprobablyofhigherantiquity。
THEUSPALLATARANGE。
TheroadbytheVillaVicencioPassdoesnotstrikedirectlyacrosstherange，butrunsforsomeleaguesnorthwardalongitswesternbase：andI
mustbrieflydescribetherockshereseen，beforecontinuingwiththecolouredeastandwestsection。AtthemouthofthevalleyofCanota，andatseveralpointsnorthwards，thereisanextensiveformationofaglossyandharsh，andofafeldspathicclay-slate，includingstrataofgrauwacke，andhavingatortuous，nearlyverticalcleavage，traversedbynumerousmetalliferousveinsandothersofquartz。Theclay-slateisinmanypartscappedbyathickmassoffragmentsofthesamerock，firmlyrecemented;
andbothtogetherhavebeeninjectedandbrokenupbyverynumeroushillocks，rangingnorthandsouth，oflilac，white，darkandsalmon-
colouredporphyries：onesteep，nowdenuded，hillockofporphyryhaditsfaceasdistinctlyimpressedwiththeanglesofafragmentarymassoftheslate，withsomeofthepointsstillremainingembedded，assealing-waxcouldbebyaseal。AtthemouthofthissamevalleyofCanota，inafineescarpmenthavingthestratadippingfrom50to60degreestotheN。E。
(Nearlyoppositetothisescarpment，thereisanothercorrespondingone，withthestratadippingnottotheexactlyoppositepoint，orS。W。buttoS。S。W。：consequentlythetwoescarpmentstrendtowardseachother，andsomemilessouthwardtheybecomeactuallyunited：thisisaformofelevationwhichIhavenotelsewhereseen。)，theclay-slateformationisseentobecoveredby——(1st)apurple，claystoneporphyryrestingunconformablyinsomepartsonthesolidslate，andinothersonathickfragmentarymass;
(2nd)，aconformablestratumofcompactblackishrock，havingaspheroidalstructure，fullofminuteacicularcrystalsofglassyfeldspar，withredspotsofoxideofiron;(3rd)，agreatstratumofpurplish-redclaystoneporphyry，aboundingwithcrystalsofopaquefeldspar，andlaminatedwiththin，parallel，oftenshort，layers，andlikewisewithgreatirregularpatchesofwhite，earthy，semi-crystallinefeldspar;thisrock(whichI
noticedinotherneighbouringplaces)perfectlyresemblesacuriousvarietydescribedatPortDesire，andoccasionallyoccursinthegreatporphyriticconglomerateformationofChile;(4th)，athinstratumofgreenishwhite，induratedtuff，fusibleandcontainingbrokencrystalsandparticlesofporphyries;(5th)，agrandmass，imperfectlycolumnaranddividedintothreeparallelandcloselyjoinedstrata，ofcream-colouredclaystoneporphyry;(6th)，athickstratumoflilac-colouredporphyry，whichIcouldseewascappedbyanotherbedofthecream-colouredvariety;Iwasunabletoexaminethestillhigherpartsoftheescarpment。Theseconformablystratifiedporphyries，thoughnoneareeithervesicularareamygdaloidal，haveevidentlyflowedassubmarinelavas：someofthemareseparatedfromeachotherbyseamsofinduratedtuff，which，however，arequiteinsignificantinthicknesscomparedwiththeporphyries。Thiswholepileresembles，butnotveryclosely，someofthelessbrecciatedpartsofthegreatporphyriticconglomerateformationofChile;butitdoesnotprobablybelongtothesameage，astheporphyrieshererestunconformablyonthealteredfeldspathicclay-slate，whereastheporphyriticconglomerateformationalternateswithandrestsconformablyonit。Theseporphyries，moreover，withtheexceptionoftheoneblackishstratum，andoftheoneindurated，whitetufaceousbed，differfromthebedscomposingtheUspallatarangeinthelineoftheVillaVicencioPass。
