54 H. E. Gregory — A Geologic Reconnaissance 



naissance examination eight groups of strata have been 

 tentatively outlined. For purposes of description these are 

 classified as follows : 



Recent — Gravels, sands, travertine. 



Pleistocene — Glacial gravels; moraines; lacustrine deposits. 



Pleistocene-Pliocene — San Sebastian formation : Lacustrine sands, 

 clays, limestones. 



Tertiary (?) — Volcanic rocks: Andesite and basalt. Bambanusa 

 formation : White and pink friable quartz sandstones. 



Upper (?) Cretaceous — Yucay formation: Blue-gray brecciated 

 limestone. 



Jura-Trias (?) — Huayllabamba formation: Bed and brown thin- 

 bedded sandstone and arenaceous shales. Molle-Orcco con- 

 glomerate member : Coarse sandstone and conglomerate. 

 Pachatucsa formation : Brown volcanic conglomerates and 

 sandstones. 



Permian (?) — Qquilque formation: Brown and gray sandstones, 

 limestone conglomerate, and gypsum. 



The age of only one of these stratigraphic groups, the Yucay 

 formation, is directly proved by paleontologic evidence. The 

 presence of characteristic glacial deposits is the basis for the 

 assignment of the great accumulations of undisturbed gravels 

 to the Pleistocene. The date of deposition assigned to the 

 lacustrine sediments constituting the San Sebastian formation 

 rests on physiographic relations, supplemented to a slight 

 degree by evidence from fossils. The position in the time 

 scale of the Bambanusa, Huayllabamba, Pachatucsa, and Qquil- 

 que formations has not been satisfactorily determined. The 

 basis of subdivision is lithologic dissimilarity, little known struc- 

 tural relations, and a meager knowledge of the regional geology 

 of southern Peru. The shales in the basal beds resemble in 

 certain respects the Carboniferous (Pennsylvania) strata of 

 Lake Titicaca, 23 but coal, massive limestones, and heavily bedded 

 gray sandstones are absent, and marine fossils, so abundant at 

 Titicaca Island and Copacobana, were not found at Cuzco. It 

 is not improbable that the formations provisionally classed as 

 Permian (?) and Jura-Trias (?) may have originated in one 

 geologic period, and the system to which they are ultimately 

 assigned may be any part of the stratigraphic column from 

 Carboniferous to Lower Cretaceous. 



23 Gregory, H. E., G-eological sketch of Titicaca Island and adjoining 

 areas, this Journal, xxxvi, pp. 187-213, 1913. 



