Hoy on a long walk round the head of the harbour. We
were eleven hours without tasting any water, and some of
the party were quite exhausted. From the summit of a hill
(since well named Thirsty Hill) a fine lake was spied, and two
of the party proceeded with concerted signals to show whether
it was fresh water. What was our disappointment to find a
snow-white expanse of salt, crystallized in great cubes ! We
attributed our extreme thirst to the dryness of the atmosphere
; but whatever the cause might be, we were exceed-
mgly glad late in the evening to get back to the boats.
Although we could nowhere find, during our whole visit, a
single drop of fresh water, yet some must ex ist; for by an odd
chance I found on the surface of the salt water, near the head
of the bay, a Colymbetes not quite dead, which in all probability
had lived in some not far distant pool. Three other kinds
of insects,—a Cincindela, like hybrida, Cymindis, and a Flar-
palus, which all live on muddy flats occasionally overflowed by
the sea, and one other beetle found dead on the plain,—completes
the list of coleóptera. A good-sized fly (Tabanus) was
extremely numerous, and tormented us by its painful bite.
The common horsefly, which is so troublesome in the shady
lanes of England, belongs to this genus. We here have the
puzzle, that so frequently occurs in the case of musquitoes ;
on the blood of what animals do these insects commonly
feed ? The guanaco is nearly the only warmblooded quadruped,
and they are present in numbers quite inconsiderable,
compared to the multitude of flies.
The foundation of porphyry is not here present, as it was at
Port Desire, and in consequence the tertiary deposits are arranged
with greater regularity. Five successive plains of different
altitudes are very distinct. The lower one is a mere
fringe nearly on a level with the sea, but the upper one is
elevated 950 feet. This latter is represented in this neighbourhood
only by a few truncate conical hills, of exactly the same
height. It was very interesting to stand on one of these flat
patches of gravel, and viewing the wide surrounding country,
to speculate on the enormous quantity of matter which must
have been removed, thus to leave these mere points, as mea-
ures of the former taWe-land.
I will now give a brief sketch of the geology of the grand
tertiary formation of Patagonia, which extends from the
Strait of Magellan to the Bay of S. Antonio. In Europe,
deposits of the more recent eras have generally been accumulated
in small basins or trough-shaped hollows. In South
America, however, the entire plains of Patagonia extending
seven hundred miles in length, and backed on the one hand
by the chain of the Andes, and fronted on the other by the
shores of the Atlantic, are thus constituted. Moreover the
northern boundary is merely assumed in consequence of a
mineralogical change in the strata : if organic remains were
present, it probably would be found to be only an artificial
limit. Again to the northward (1300 miles distant from the
Strait of Magellan) we have the Pampas deposit, which
though very different in composition, belongs to the same
epoch with the superficial covering of the plains of Patagonia.
The cliffs on the coast give the following section : The
lower part consists of a soft sandstone, containing large concretions
of a harder nature. These strata contain many
organic remains—immense oysters nearly a foot in diameter,
curious pectens, echini, turritellæ, and other shells, of which
the greater portion are extinct, but a few resemble those now
existing on the coast.* Above these fossiliferous beds, a
mass of soft friable stone or earth is superimposed, which,
from its extreme whiteness, has been mistaken for chalk. It is,
however, quite different ; and closely resembles the less argillaceous
varieties of decomposed felspar. This substance
never contains organic remains. Lastly, the cliff is surmounted
by a thick bed of gravel, almost exclusively
derived from porphyritic rocks. For the sake of making
the following description more easily intelligible, I have
subjoined an imaginary section of the plains near the coast.
* T h e g e o lo g is t m u s t re c o lle c t th is is a m e re s k e tc h , a n d th a t th e fossil
sh e lls h av e n o t y e t b e e n c a re fu lly ex am in e d .
I