Silurian System. 43 



vince of Chiquitos, porphyritic or granitoidal gneisses, which 

 are superimposed on granite, and form the support of line 

 grained gneisses, or of mica-slates containing garnets and 

 gnenatites; at Monte- Video, and at Maldonado, there are 

 blackish and finely laminated gneisses ; and at Tandil, ac- 

 cording to the determination of M. Cordier, there are tabular 

 varieties of petrosilex. 



In Brazil, and in the east of the province of Chiquitos, the 

 gneiss everywhere supports transition clay-slates. But when 

 the latter are awanting, the gneiss is succeeded by formations 

 of a much more modern date ; for at Conception, at San 

 Ignacio, and at Santa Anna de Chiquitos, portions of the 

 Patagonian territory formation repose on the gneiss. M. Pissio 

 has pointed out tertiary deposits, resembling the molasse of 

 Europe, reposing on the gneiss of the environs of Bahia. At 

 Monte-Video, and in the Pampas, the gneiss is surrounded by 

 the Pampian tertiary formation ; and, lastly, at Chiquitos, it 

 is covered by the recent alluvia. 



Silurian System. — The most ancient beds which M. d*Or- 

 bigny has found superimposed on the rocks of a decidedly 

 crystalline character in South America, present, wherever he 

 has seen them, a very uniform composition. They are, in 

 their lower portions, coarse clay-slates {des phyUades sckis- 

 toides)y having a blue colour, and often containing chiastolite ; 

 and they pass in their middle portions into fine grained clay- 

 slates (des phy Hades satin^s), having a rose colour. These 

 two series of beds, which are the most developed, possessing 

 frequently a thickness of several hundred yards, do not con- 

 tain any traces of organized beings. Above these there are 

 sandstone flags (des phy Hades yresiformes), or very micaceous 

 slaty sandstones (yres phylladifires tres micacis), whose thick- 

 ness is upwards of 160 feet. 



In these last mentioned beds, M. d'Orbigny has collected 

 fossils, which, however, are rare, and which belong to the 

 genera Cruziana, Orthis, Lingular Calymene, Asaphus, and 

 Graptolithus. Of ten species of these genera, eight are ex- 

 tremely analogous to the species of the silurian strata of 

 Europe, and three^ viz., Calymene macrophthalnia^ Cruziana 

 rugosa, and Graptolithus dentatus, are even identical with 



