44 Geology of South America. 



them. We may say, therefore, that these fossils, as a whole, 

 have the same aspect, the soxne fades ^ as those of the Silurian 

 formations of Europe. Here is an identical zoological phy- 

 siognomy transported thousands of leagues. The rocks also 

 present mineralogically many analogies to those of the Silu- 

 rian formations of Europe. This double relation, conjoined 

 with the position these formations occupy below all the other 

 fossiliferous deposits of South America, has naturally led M. 

 d'Orbigny to refer them to the Silurian system, as established 

 by Mr Murchison ; and it is probable that they are at least 

 very nearly connected with it. 



These Silurian formations of South America occupy spaces 

 of considerable magnitude, and occur at points very re- 

 mote from one another. They present themselves along 

 nearly the whole eastern border of the Bolivian plateau, 

 forming a band which follows the Andes, properly so called, 

 or the eastern Cordillera, and is parallel to the granitic rocks, 

 from Sorata to Illimani, a distance of more than 300 English 

 miles. To the east of the eastern Cordillera they are still 

 more developed, and form a band of nearly 40 miles in 

 breadth, and upwards of 600 miles in length, included between 

 the plains of Santa Cruz de la Sierra on the east, and the 72d 

 degree on the west. They thus form an immense band on 

 the east, as well as on the west of the eastern chain, and 

 which runs from N.W. to S.E., but is much more developed 

 to the east than to the west of the chain. In the region in- 

 cluded between the Andes and Brazil, we again find the Silu- 

 rian series in the south of the province of Chiquitos, near Ta- 

 pera, near San-Juan, to the north of the Sierra of Santiago, 

 and to the south of that of Sunsas. They there constitute a 

 band running from E.S,E. to W.N.W., and having a length of 

 upwards of 150 miles. There, as in the Andes, they present, 

 at their lower portion, a blue coarse slate, supporting fine 

 rose-coloured clay-slates, on which repose yellowish slates. 

 M. d'Orbigny was, however, unable to find any trace of or- 

 ganized bodies in these beds, of which the first has a thick- 

 ness of at least 600 feet, while the others are not more than 

 50 or 60 feet thick. 



In Bolivia, the Silurian strata possess an interest of a very 



