Iwillnowgive，first，asketchofthestructureoftherange，asrepresentedinthesection，andwillthendescribeitscompositionandinterestinghistory。Atitswesternfoot，ahillock[N]isseentoriseoutoftheplain，withitsstratadippingat70degreestothewest，frontedbystrata[O]inclinedat45degreestotheeast，thusformingalittlenorthandsouthanticlinalaxis。Someotherlittlehillocksofsimilarcomposition，withtheirstratahighlyinclined，rangeN。E。andS。W。obliquelytothemainUspallataline。Thecauseofthesedislocations，which，thoughonasmallscale，havebeenviolentandcomplicated，isseentolieinhummocksoflilac，purpleandredporphyries，whichhavebeeninjectedinaliquifiedstatethroughandintotheunderlyingclay-slateformation。Severaldykeswereexposedhere，butinnootherpart，thatI
sawofthisrange。Asthestrataconsistofblack，white，greenishandbrown-colouredrocks，andastheintrusiveporphyriesaresobrightlytinted，amostextraordinaryviewwaspresented，likeacolouredgeologicaldrawing。Onthegentlyinclinedmainwesternslope[PP]，abovethelittleanticlinalridgesjustmentioned，thestratadipatanaverageangleof25
degreestothewest;theinclinationinsomeplacesbeingonly19degrees，insomefewothersasmuchas45degrees。Themasseshavingthesedifferentinclinations，areseparatedfromeachotherbyparallelverticalfaults[asrepresentedatPa]，oftengivingrisetoseparate，parallel，uniclinalridges。Thesummitofthemainrangeisbroadandundulatory，withthestratificationundulatoryandirregular：inafewplacesgraniticandporphyriticmasses[Q]protrude，which，fromthesmalleffecttheyhavelocallyproducedinderangingthestrata，probablyformtheupperpointsofaregular，greatunderlyingdome。Thesedenudedgraniticpoints，I
estimatedataboutninethousandfeetinheightabovethesea。Ontheeasternslope，thestrataintheupperpartareregularlyinclinedatabout25degreestotheeast，sothatthesummitofthischain，neglectingsmallirregularities，formsabroadanticlinalaxis。Lowerdown，however，nearLosHornillos[R]，thereisawell-markedsynclinalaxis，beyondwhichthestrataareinclinedatnearlythesameangle，namelyfrom20to30degrees，inwardsorwestward。Owingtotheamountofdenudationwhichthischainhassuffered，theoutlineofthegentlyinclinedeasternflankscarcelyofferstheslightestindicationofthissynclinalaxis。Thestratifiedbeds，whichwehavehithertofollowedacrosstherange，alittlefurtherdownareseentolie，Ibelieveunconformably，onabroadmountainousbandofclay-slateandgrauwacke。Thestrataandlaminaeofthislatterformation，ontheextremeeasternflank，aregenerallynearlyvertical;furtherinwardstheybecomeinclinedfrom45to80degreestothewest：nearVillaVicencio[S]
thereisapparentlyananticlinalaxis，butthestructureofthisouterpartoftheclay-slateformationissoobscure，thatIhavenotmarkedtheplanesofstratificationinthesection。OnthemarginofthePampas，somelow，muchdislocatedspursofthissameformation，projectinanorth-