Devonian Si/ stem. 45 



direct kind, inasmuch as tliey contain the richest of the Boli- 

 vian gold mines, and also some mines of silver. Wherever 

 gold is met with in situy it occurs in veins of milky quartz, 

 which traverse the lower beds, viz. the coarse clay-slates. It 

 is in such a geological position, that the ore is mined on the 

 slopes of Illimani, at Oruro, at Potosi, &c. If we consider 

 that all the localities where gold is obtained by washing, are 

 situated in valleys where the clay-slates have been much dis- 

 located and denuded, as, for example, at the Kio de la Paz, at 

 Tipoani, at the Rio de Suri, at the Rio de Choquecamata, &c., 

 we must naturally conclude that the ore is derived from the 

 same slates. 



Devonian System. — Wherever M. d'Orbigny observed the 

 Silurian series, it was succeeded by an enormous mass of 

 hard quartzose sandstones or quartzites, which, from their 

 position and fossils, he regards as the representative of the De- 

 vonian formation, or the old red sandstone. This extensively 

 distributed system is composed of whitish or yellowish com- 

 pact quartzose sandstones, without traces of fossils, passing, 

 in their lower portions, into blackish or ferruginous, very mica- 

 ceous, slaty sandstones, and only then containing the remains 

 of organized bodies, sometimes in large beds, at other times 

 disseminated through the strata of rock. These rocks are 

 almost always superimposed on the Silurian series, and most 

 frequently in a conformable manner. They are succeeded, 

 but unconformably, by carboniferous strata, which are charac- 

 terized, in a particular manner, by the fossils they contain. 



This great quartzose deposit presents itself over as large 

 spaces as the Silurian system, which it everywhere accompa- 

 nies ; and it is distributed in the same way. On each side of 

 the band of Silurian rocks of the eastern chain of the Andes, 

 it forms, for about 450 miles, another great parallel band, in- 

 dependently of the detached portions scattered through the 

 interior of the Silurian band. There is also a great develop- 

 ment of these quartzose sandstones on the Silurian formation 

 of the eastern part of the province of Chiquitos. 



Apart from his own personal observations, M. d'Orbigny 

 has learned that these same quartzose formations abound in 

 Brazil, in the chain of Parecys, in that of Diamantino, to the 

 west of Motogrosso, and in those chains which are to the east 



