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 metamor- 

 phic gneiss, of which the strata were tilted at a high angle." * 



The plateaux in which the Parana audits 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 canrja, 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 casvalho. 



Rivers — The S. Francisco. 



The Ptio 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, Notts of a Natutalist m South America, p. 317. 



