The universal geography : earth and its inhabitants

S. FEANCISCO BASIN. 155
geological and pre-historic records of Brazil. The limestone district is pierced by-
innumerable caves, some mere fissures, others vast galleries, huge vaulted chambers,
winding passages, ramifying in an endless maze of underground recesses. The
rocks seem to have been first crushed by tremendous lateral pressure, and then
eroded by running waters. Calcareous concretions hang from the vaults of the
caverns, or rise in pillars from the floor, which is covered with argillaceous layers
of varying thickness containing land and fresh-water shells identical with contem-
porary species. In these layers have also been found enormous quantities of animal
remains which have been studied by Claussen, and later more successfully by
Lund.
East of the S. Francisco valley the " Backbone " consists mainly of gneiss,
passing in certain places to granite, syenite, and mica-schist. The crystalline
rocks are of a granulated texture, with large feldspar crystals easily disintegrated
and forming arenaceous and reddish layers disposed in broad slopes at the base of
the hills ; in some districts these layers, covered with a vegetable humus, are nearly
1,000 feet thick. Nowhere are seen any sedimentary deposits overlying masses of
gravel produced by the disintegration of mountains, which at one time stood at a
prodigious elevation above sea level. " The conclusion is irresistible that ancient
Brazil was one of the greatest mountain regions of the earth, and that its summits
may very probably have exceeded in height any now existing in the world. What
we now behold are the ruins of the ancient mountains, and the singular conical
peaks are, as Liais has explained, the remains of some harder masses of metamorphic
gneiss, of which the strata were tilted at a high angle." *
The plateaux in which the Parana and its affluents have excavated their upper
valleys are formed to a considerable, but still undetermined, depth of the tritu-
rated fragments of the ancient Brazilian highlands ; such is also the origin of the.
plains of Paraguay Gran Chaco, and the Argentine pampas, as well as of the
sandbanks in the Plata estuary. In this chemical laboratory the rocks have
changed their place and form—from crystalline mountains they have become
stratified plains.
Here also the ground contains much gold, as well as iron ores, and, in some
districts, diamonds. Those mines more especially are worked which are covered
with caiiga, a recent conglomerate formed by the detritus of the mountains, and
cemented by ferruginous waters. The gravels under which diamonds are found
are known by the name of cascalho.
Rivers—The S. Francisco.
The Rio S. Francisco, explored by Halfeld in 1852-54, and by Liais in 1862,
was known in its higher reaches to the Paulistas before its lower course had been
traced or identified with the estuary discovered and named the S. Francisco in the
year 1501.
After flowing for about half its course from south to north parallel with the
Tocantins, Xingu, and other Amazons affluents, it trends round to the north-east
* John Ball, ^oles of a Naturalist m South America, p. 317.