Ayusbamba (Peru) Fossil Beds. 135 



Bed rock. — The lake beds at Ayusbamba rest on sandstone 

 coated with a layer of coarse bowlders and followed upwards 

 by clays, sands, ash and peat, above which, near the edges of 

 the basin, are unstratified angular gravels distributed as fans 

 and surface wash. The rock floor is exposed on the north 

 and on the south rims of the basin and in the beds of several 

 of the minor stream branches. The bed rock is essentially 

 sandstone, brown or grey in tone, especially on weathered sur- 

 faces. The strata, which vary from 6 inches to 20 feet in 

 thickness, rarely maintain their individuality for more than 100 

 feet along the strike, and may be considered as a series of 

 lenses whose dimensions and texture are subject to abrupt 

 changes. Angular cross bedding is a noticeable feature in the 

 medium-grained sandstone, and sun-baked surfaces were 

 observed on the bedding planes of the shaly limestone. A 

 number of minute faults were noted. 



While the rock ledges underlying and surrounding the 

 ancient lake basin may roughly be grouped as sandstones, there 

 is considerable variation in texture and composition as indicated 

 by the following rock types represented : 



(1) Sandstone, very tine, even-grained, composed of quartz 

 grains of microscopic size embedded in silicious cement. 



(2) Sandstone, medium to coarse-grained, consisting of sub- 

 angular and angular fragments of white quartz, feldspar crys- 

 tals, diorite-porphyry, sandstone, and rare limestone ; bound 

 together by silica. 



(3) Thin lenses of purple-brown, arenaceous, shaly lime- 

 stone. 



(4) Conglomerate with pebbles the size of buck shot and 

 upwards. 



The finer-grained phases of the conglomerate grade into 

 sandstones and consist of the same materials ; but in the ledges 

 facing the Chipura the pebbles are larger (1/4" to 3" in 

 diameter ; one dacite bowlder measured 2 ; ), and are distributed 

 as broken strings or as individuals embedded in a cubic foot or 

 cubic yard of sandstone matrix. The larger pebbles compos- 

 ing the conglomerate at this locality include quartzite, biotite- 

 granite with orthoclase phenocrysts and a dark grey igneous 

 rock with granitic and porphyritic facies. The last-named 

 rock which is not only a constituent of the conglomerate but is 

 present in the overlying bowlder bed and occurs in fragments 

 of various sizes clotting the surface southwest of the 

 hacienda buildings, is worthy of special notice. A hand speci- 

 men of this rock is seen to consist essentially of crystals of 

 hornblende, •§•" to y in length, embedded in a groundmass of 

 crystals of microscopic size. A few pyroxene crystals are 

 also present as phenocrysts. Microscopic examination of the 



