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 alterate very many times, and are
quite conformable one to the other. During two days of careful examination
I said to myself at least fifty times, how exactly like (only rather
harder) these beds are to those of the upper Tertiary strata of Patagonia,