The Cordillera in this part consists of two principal ranges, the passes across which attain respectively an elevation of 13,210 and 14,365 feet. The first great line (consisting, of course, of many subordinate ones) is called Peuquenes. It divides the waters of Chile and Argentina. To the eastward, a mountainous and elevated region separates it from the sec ond range (called the Portillo) overlooking the Pampas (see article SOUTH AMERICA — Geology).
The geological structure of the mountains, at this point of observation, is very briefly sketched, and first that of the Peuquenes or western line; for the constitution of the two ranges is totally different. The lowest strati fied rock is a dull red or purple claystone porphyry, of many varieties, alternating with conglomerates, and breccia composed of a similar substance; this formation attains a thickness of more than a mile. Above it there is a grand mass of gypsum, which alternates with, passes into, and is replaced by red sand stone, conglomerates and black calcareous clay slate. Even at the very crest of the Peuquenes, at the height of 13,210 feet, the black clay-slate contains numerous marine remains, amongst which a gryphwa is the most abundant, like wise shells, resembling turritellm, terebratulae and an ammonite. The formation probably is of the age of the central parts of the secondary series of Europe. These great piles of strata have been penetrated, upheaved and over turned, in the most extraordinary manner, by masses of injected rock, equaling mountains in size. On the bare sides of the hills, compli cated dikes and wedges of variously-colored porphyries and in stones, are seen travers ing the strata n every possible form and di rection; proving also, by their intersections, successive periods of violence. The rock which composes the axis of these great lines of dislo cation when viewed at a distance resembles granite, but on examination it is found rarely to contain any quartz; and instead of ordinary feldspar, albite. The metamorphic action has been very great, as might have been expected from the close proximity of such grand masses of rock, which were injected when in a lique fied state from heat. When it is known, first, that the stratified porphyries have flowed as streams of submarine lava under an enormous pressure, and that the mechanical beds separat ing them owe their origin to explosions from the same submarine craters; secondly, that the whole mass in the lower part has generally been so completely fused into one solid rock by metamorphic action that the lines of di vision can only be traced with much difficulty; and thirdly, that masses of porphyry, unchs tinguishable by their mineralogical characters from the two first kinds, have been subsequently injected ;— the extreme complication of the whole will readily be believed.
We now come in these observations made in the south-central division of the Andes to the second range, which is of even greater altitude than the first. Its nucleus in the sec tion seen by Mr. Darwin when crossing the •Portillo Pass consists of magnificent pinnacles of coarsely-crystallized red granite. On the eastern flank a few patches of mica slate still adhere to the unstratified mass; and at the foot a stream of basaltic lava has burst' forth at some remote period,— perhaps when the sea covered the wide surface of the Pampas. On the western side of the axis, between the two ranges, laminated fine sandstone has been penetrated by immense granite dikes proceed ing from the central mass, and has thus been converted into granular quartz rock. The sandstone is covered by other sedimentary de posits, and these again by a coarse conglomerate of vast thickness. All these coarse mechanical beds dip from the red granite directly toward the Peuquenes range, as if they passed beneath it ; though such is not the case. Examination of the pebbles composing this conglomerate (which betray no signs of metamorphic ac tion) disclose perfectly rounded masses of the black calcareous clay-slate with organic re mains,— the same rock which is found on the Peuquenes. These phenomena compel us to arrive at the following conclusion : That the Peuquenes existed as dry land for a long period anterior to the formation of the second range, and that, during this period, immense quantities of shingle were accumulated at its submarine flank. The action of a disturbing force then commenced: These more modern deposits were injected by dikes, altered by heat, and tilted toward the line whence, in the form of sedi ment and pebbles, they had originally pro ceeded,—thus making the offspring at first appear older than its parent. This second, grand and subsequent line of elevation is parallel to the first and more ancient one.