of the Cuzco Valley, Peru. 59 



glomerate 20 feet thick consisting of limestone and gypsum 

 balls, intersected by seams of gypsum. In places the sandstone 

 of the Qquilque formation is coarse grained or conglomeratic, 

 and at a point three-fourths of a mile northeast of Ttica-Ttica 

 Pass the upturned beds of sandstone contain pellets of shale, 

 subangular pebbles of quartz exceeding half an inch in diam- 

 eter, and angular fragments of" scoriaceous basalt of much 

 larger dimensions. 



The age of the Qquilque strata is unknown; their assign- 

 ment to the Permian rests on imperfectly determined field 

 relations. That these beds are continental rather than marine 

 in origin is suggested by various features described above. 



Pachatucsa Formation. 



The volcanic conglomerates and sandstones forming the 

 heights of Pachatucsa and Atasccasa, on the northern rim of 

 the Cuzco Valley, are grouped in the Pachatucsa formation. 

 The strata constitute part of the south limb of a giant anticline 

 whose crest has been trenched by Urubamba River to a depth 

 of nearly 6,000 feet. Sierra Pachatucsa has thus been carved 

 into a cuesta whose bluntly serrated crest, attaining a height 

 of 15,915 feet, constitutes the commanding topographic fea- 

 ture of the bordering walls of the Cuzco Valley. Prom the 

 summit of the cuesta the descent to the Urubamba is precipi- 

 tous; toward the south a dip slope at an angle of 20° to 30°, 

 gouged by ancient glaciers, extends for more than a mile 

 (fig. 10). 



In a broad sense the strata included in the Pachatusca 

 formation may be classed as volcanic conglomerate. They 

 exhibit, however, wide variations in amount, character, and 

 distribution of the igneous constituents. In the Pachatucsa 

 hogback, where the tilted beds of dark-red conglomerate are 5 

 to 40 feet thick, the lowest strata examined are composed largely 

 of quartz pebbles; the upper beds include angular and sub- 

 angular fragments of quartz, sandstone, quartzite, basalt, and 

 several varieties of dense basic rock embedded in a matrix 

 of similar material. Six pebbles were collected in the field 

 as representatives of different types of igneous rock. In color 

 the specimens are brown, red, greenish, white, drab, and 

 mottled; their textures range from scoriaceous through por- 

 phyritic to microcrystalline. When the pebbles were subjected 

 to microscopic analyses, however, they were all found to belong- 

 to the andesite family. They consist of andesine as pheno- 



