The universal geography : earth and its inhabitants

THE PATAGONIAN STEPPE. 3G5
The Patagoxian Steppe Lands.
The interior of the Patagonian districts watered bj' the Rios Colorado and
Kegro bristles with rocky crests, pink porphyries, aud granites, which look in the
morning siin like lightly-tiuted vapours. These various groups, known by the
general name of mahuida, that is, " mountains" in the native language, have an
average height of from 1,300 to 1,650 feet, and are nearly all disposed north-
west and south-east, like the chains of hills between Buenos Ayres and Bahia
Blanca.
Between the ridges the ground is strewn with rolled pebbles, granites, gneiss,
porphyries disposed in horizontal layers alternating with the dunes. These beds
of rolled gravels cover all the tertiary plains which constitute the whole of the
Patagonian plateau east of the Andes, aud which contain a superabundance of
fossil remains. This prodigious mass of Patagonian gravels was calculated by
Darwin to extend for about 600 miles north and south, with a mean breadth of
200 miles, and a depth of 50 feet. Whole mountain ranges must have been
triturated to yield such gravel beds as these ; and to the Patagonian deposits must
be added the detritus of like nature at present covering the marine bed itself.
Such are the rolled porphyries which the soundings have fished np in the waters
of the Falkland Islands, far from any insular masses containing analogous rocks.
These pebbles are evidently derived from the Andes and the older mountains,
which formerly rose above the central aud eastern plains, and of which nothing
now remains except the nuclei. Glacial moraines have undoubtedly supplied the
raw material, which has been distributed by the marine waters in horizontal or
very slightly inclined beds. Then followed the phenomenon of emersion, due
either to an upheaval of the land, or to a subsidence of the sea. Thus the old
shingly beach became the dry gravel pits of Patagonia, in which are found pro-
digious qiiantities of those gigantic oysters, 15 to 20 inches round, which are so
wide!}' diffused throughout the soil of Patagonia. Near Possession Bay, at the
Atlantic entrance of Magellan Strait, de Pourtales discovered a lagoon standing
160 feet above sea level, and containing shells absolutely identical with those of
the neighbouring waters.
Hence there can be no reasonable doubt as to the general movement of
upheaval along the Patagonian seaboard. But geologists have not yet determined
its true character, and while some suppose that it took place in a succession of
sudden upward thrusts, corresponding to the several raised terraces, others with
more probability suggest that it was, on the contrary, a slow movement produced
in a series of rhythmical undulations.
During the contemporary period other formations have been superimposed on
the Patagonian gravel beds, and on the argillaceous clays of Central Argentina.
Over vast spaces the ground is now covered with sands, which form dunes
analogous to those developed on many coastlands under the influence of the winds
setting from the high seas. But in the Platean regions these shifting dunes are
not of marine origin ; they are, on the contrary, derived from the region of foot-