retions of a crystalline marble of a blackish grey
colour. The upper beds which form some of the higher pinnacles consist
of layers of snow-white gypsum and red compact sandstone, from the
thickness of paper to a few feet, alternating in an endless round.
The rock has a most curiously painted appearance. At the pass of the
Peuquenes in this formation, where however a black rock like clay-slate,
without many laminae, occurring with a pale limestone, has replaced the
red sandstone, I found abundant impressions of shells. The elevation
must be between 12 and 13,000 feet. A shell which I believe is the
Gryphaea is the most abundant--an Ostrea, Turratella, Ammonites, small
bivalves, Terebratulae (?). Perhaps some good conchologist (6/1. Some of
these genera are mentioned by Darwin ("Geol. Obs." page 181) as having
been named for him by M. D'Orbigny.) will be able to give a guess, to
what grand division of the formations of Europe these organic remains
bear most resemblance. They are exceedingly imperfect and few. It
was late in the season and the situation particularly dangerous for
snow-storms. I did not dare to delay, otherwise a grand harvest might
have been reaped. So much for the western line; in the Portillo pass,
proceeding eastward, we meet an immense mass of conglomerate, dipping to
the west 45 deg, which rest on micaceous sandstone, etc., etc., upheaved
and converted into quartz-rock penetrated by dykes from the very
grand mass of protogine (large crystals of quartz, red feldspar, and
occasional little chlorite). Now this conglomerate which reposes on and
dips from the protogene 45 deg consists of the peculiar rocks of the
first described chain, pebbles of the black rock with shells, green
sandstone, etc., etc. It is hence manifest that the upheaval (and
deposition at least of part) of the grand eastern chain is entirely
posterior to the western. To the north in the Uspallata pass, we have
also a fact of the same class. Bear this in mind: it will help to
make you believe what follows. I have said the Uspallata range is
geologically, although only 6,000-7,000 feet, a continuation of the
grand eastern chain. It has its nucleus of granite, consists of grand
beds of various crystalline rocks, which I can feel no doubt are
subaqueous lavas alternating with sandstone, conglomerates and white
aluminous beds (like decomposed feldspar) with many other curious
varieties of sedimentary deposits. These lavas and sandstones alter