of the Cuzco Valley, Peru. 77 



surfaces the rock is white to gray in color; pink tones result- 

 ing from the presence of pink quartz fragments and a small 

 amount of ferric cement characterize the fresh rock. Portions 

 of the ledges studied are fine-grained, but the typical sandstone 

 members are coarse and gritty and consist of subangular, clear 

 quartzes and fragments of decomposing feldspar. The con- 

 glomeratic phases are displayed as thin lenses or narrow 

 stringers and consist of partly rounded, usually elongated 

 pebbles of quartz, quartzite, and sandstone, rare igneous frag- 

 ments, pellets and slabs of shale, and crystals of feldspar 

 which retain their original form. The pebbles range from a 

 fraction of an inch to over 2 inches in diameter and are 

 arranged without evident sorting. The rock as a whole is 

 firmly cemented with silica and iron, some parts in fact being 

 developed as a glistening pink quartzite. The strata are 

 highly cross-bedded, a structural feature exhibited both by 

 curved bands and straight lines in the sandstone members and 

 even more markedly by lenses and beds of conglomerate and 

 coarse sandstone 6 to 10 feet in thickness. Unlike the Bam- 

 banusa sandstones south of Ichchu-Orcco, the beds on the 

 divide, doubtfully assigned to this formation, dip southwest, 

 in harmony with the strata of the Pachatucsa formation. A 

 continental origin for the Bambanusa formation is suggested 

 by .the structural features already described, but the geologic 

 period during which the depositing streams were active is not 

 known. Superficially the formation appears to be geologically 

 young, perhaps early or middle Tertiary, and the field relations 

 suggest that its component strata were laid down as sands and 

 gravels in a basin developed by warping or faulting of the 

 underlying older strata. On the other hand the absence of the 

 Yucay limestone (Upper (?) Cretaceous) from the area south 

 and east of Cerro Picol may indicate that parts of the Bam- 

 banusa formation are contemporaneous with the Pachatucsa and 

 Huayllabamba formations, assigned to the Jura-Trias (?). 



Sedimentary Rocks near the Motjth of the Huatanat. 



The sedimentary rocks of the lower Cuzco Valley were not 

 mapped ; they may belong with the Huayllabamba formation. 

 The following notes were made during a rapid traverse from 

 Sierra-Bella to Huacarpay. 



The banks of the Huatanay at its junction with the Uru- 

 bamba are formed of beds of resistant brown sandstone, 4 

 inches to 1 foot in thickness, interstratified with chocolate-red 



