Ayusbamba (Peru) Fossil Beds. 137 



their origin and conditions of deposition. However, the 

 irregularity of bedding, the abrupt change in composition and 

 texture along strike, the predominance of subangular bowlders, 

 the type of cross bedding, the presence of sun-baked surfaces, 

 and the brown color of the rock suggest continental leather 

 than marine sedimentation. 



Bowlder Bed. — Directly overlying bed rock and immediately 

 beneath lacustrine clays and sands, a bowlder bed is exposed 

 at a few localities. The pebbles, all subangular, are chiefly 

 igneous, and include the following varieties : Andesite, dense 

 and scoriaceous ; diorite-porphyry ; granite-porphyry ; rare 

 granite ; black and green hornstone ; black, pink, and gray 

 quartzites ; brown and red sandstones. About eighty per cent 

 of the bowlders measure between one and six inches ; the 

 largest seen in place is four feet in diameter, although larger 

 blocks are embedded in the banks of Calvo brook. In places 

 the bowlder bed is loosely compacted ; elsewhere so firmly 

 cemented that pebbles may be broken without disturbing the 

 matrix. The exposures of bowlder bed studied gave no cer- 

 tain clue as to origin. The heterogeneous character of the 

 mass, both as regards composition and stratification, suggest 

 morainal accumulations, yet striated bowlders and scoured floor 

 were not observed. Part of the material might well have 

 been supplied as talus, but with the exception of sandstone 

 fragments the bowlders are of types not represented in the 

 country rocks. The history of the bowlder beds cannot be 

 written on the basis of present knowledge of the Apurimac 

 valley region. 



Lacustrine Beds (Ma/p, fig. 8). — The lake beds proper — 

 clay, fine sand, ash, and peat — rest in some places upon the 

 bowlder bed ; elsewhere directly upon the sandstone floor. 

 The strata dip slightly toward the center of the basin, but with 

 many variations and exceptions. The northward dip is in 

 general greater and suggests a tilting of the beds at a time 

 postdating their deposition. The stratigraphic succession of 

 the lacustrine beds is shown in the following selected sections : 



Section I. West branch Calvo brook. 



200' from junction. Dip of clay beds NE/ 10°. 



Feet 



1. Grass-covered slope of superficial debris 3 



Unconformity. 



2. Sand, fine, with lenses of coarse sand and of adobe, con- 



taining plant roots; a few thin clay layers 30 



3. Volcanic ash, excessively fine, soft, pure white, regu- 



larly stratified in beds about one-tenth of an inch 

 thick 1 



