Fossils in the Amazon Yalley. 283 



• 



as determined by W. M. Gabb, Esq., of Philadelphia, are : 

 Neritina jpwpa, Turhonilla miiiiiSGula^ Mesalia Ortoni, 

 Tellina Amazonensis, Pachydon obliqua^ and P. tenuaJ^ 

 All of these are new forms excepting the first, and the last 

 is a new genus. It is a singular fact that the Neritina is 

 now living in the West India waters, and the species found 

 at Pebas retains its peculiar markings. So that we have 

 some ground for the supposition that not many years ago 

 there was a comiection between the Caribbean Sea and the 

 Upper Amazon ; in other words, that Guiana has only very 

 lately ceased to be an island. There is no mountain range 

 on the water-shed between the Orinoco and the Negro and 

 Japura, but the three rivers are linked by natural canals.f 

 Interstratified with the clay deposit are seams of a highly 

 bituminous lignite ; we traced it from near the mouth of 

 the Curaray on the Pio !N"apo to Loreto on the Maranon, a 

 distance of about four hundi-ed miles. It occurs also at Iqui • 

 tos. This is farther testimony against the glacial theory of 

 the formation of the Amazonian Yalley. The paucity of 

 shells in such a vast deposit is not astonishing. It is as re- 

 markable in the similar accumulation of reddish argilla- 

 ceous earth, called " Pampean mud," which overspreads the 

 Pio Plata region.:]: Some of the Pampa shells, like those 

 at Pebas, are proper to brackish water, and occur only on 

 the highest banks. The Pampean formation is believed by 

 Mr. Darwin to be an estuary or delta deposit. We will men- 

 tion, in this connection, that silicified wood is found at the 



* These interesting fossils are figured and described in the J.?/<. Journal of 

 Conchology. 



t "The whole basin between the Orinoco and the Amazon is composed 

 of granite and gneiss, slightly covered with debris. There is a total absence 

 of sedimentary rocks. The surface is often bare and destitute of soil, the 

 undulations being only a few feet above or below a straight line." — Evan Hop- 

 kins, in Quart. Jour. Geol. Sac, vol. vi. 



X See Darmn on the absence of extensive modern conchiferous deposits iu 

 South America, Geological Observations, pt. iii,, ch. v. 



