GEOLOGY OF PERU ADAMS. 407 



of marine sponges are actually present in the deposits. Although 

 Duenas finally accepts the lacustrine theory for the deposits, he goes 

 rather far when he remarks that it is nothing wonderful to suppose 

 that Lake Titicaca once extended into the Department of Cuzco. 

 From what the writer has seen of the topography it appears alto- 

 gether improbable; and, moreover, the theory of local lakes would 

 account in a more satisfactory manner for the occurrence of the for- 

 mations. 



Tertiary of the Amazon region. 



James Orton, in his explorations of the upper Amazon Valley, 

 collected some shells from Pebas, which he submitted to Gabb, who 

 determined them (18G8) as late Tertiary. Because of the finding of 

 these shells, Orton refuted the theory of the glacial origin of the 

 clays of the Amazon basin presented by Agassiz and discussed later 

 in this report. Orton (1870) gives a description of the exposures 

 along his route of travel. He says that along the Napo Eiver the 

 only spot where the rocks are exposed is near Napo village, where 

 there is a bed of dark slate dipping east. Farther west, at the foot 

 of the Ecuadorian Andes, the prevailing rock was found by him to 

 be mica schist. The entire Napo country is covered with an alluvial 

 bed on an average 10 feet thick. The formation of the bluff near 

 Pebas he described as consisting of fine laminated clays of many 

 colors, resting on a bed of lignite or bituminous shale and a coarse 

 iron-cemented conglomerate. 



After Gabb described the collection of shells from Pebas, a larger 

 collection was made by Mr. Hauxwell, a part from Pebas but most of 

 them from 30 miles below Pebas, at Pichua. Among them Conrad 

 found (1870) seven species of Pachydon (Gabb), a genus which does 

 not have any living representative and is very different from any 

 existing fresh-water genus. He says that it is not possible to state 

 without doubt what the relative stratigraphic position of the group 

 ma}^ be, but if all the species are extinct it can not be later than Ter- 

 tiary, and that it may have lived in fresh or brackish Avater, but it is 

 certainly not of marine origin. 



A collection made by Mr. Steere at Pebas was examined by Conrad 

 (1874), who questioned there being evidence of the marine origin of 

 the shells. 



QUATERNARY. 



Pleistocene glaciation. 



OcCtTBBENCE OF SNOW PeAKS. 



Humboldt, in his personal narrative (1814), called attention to the 

 absence of snow peaks between the Nevada Huaylillas in latitude 

 7° 65' and Chimborazo in Ecuador. 

 88292— SM 1908 27 



