Ayusbamba (Peru) Fossil Beds. 139 



Notes on measured sections. — The individual clay beds are 

 nowhere thick and at all places are interbedded with fine sand. 

 At the north edge of the basin, where 135 feet of lacustrine 

 deposits are exposed, the section consists of alternating layers, 

 one-tenth of an inch to four inches thick, of fine sand and 

 banded, dark red clay in the midst of which occur three lenses 

 of coarse sand. The layers of sand and cla} 7 interleave as 

 lenses fifty to two hundred feet long. Minute folds and faults 

 further interrupt the regularity of bedding. The clays are 

 red, pink, or brown in tone, due to their content of ferric iron 

 in hydrated form, and are believed to owe their origin to the 

 decomposition of the surrounding pre-Cretaceous sandstones. 

 No "clay dogs" or concretions were observed, but a few small, 

 light-colored patches in the clays are highly calcareous in 

 marked contrast to the clay in general. Apparently the process 

 of segregation is in operation at the present time. 



The regularly banded white ash, interstratified with the clays, 

 decomposes under slight pressure to an impalpable powder 

 which has a gritty " feel." Microscopical study of this rock 

 revealed the presence of glass arranged as threads and cusps 

 and hooks, and constituting about 90 per cent of the mass ; 

 laths of plagioclase and frayed ribbons of biotite complete the 

 list of component minerals, and determine the classification of 

 the deposit as dacitic ash. 



No true peat or coal was found among the beds, but at sev- 

 eral localities thin bands of an earthy mixture of sand, clay, 

 and vegetable fragments were noted. This mixture was found 

 by microscopic examination to consist of minute shreds of 

 glass, tiny feldspar, quartz, biotite and muscovite fragments, 

 together with broken bits of volcanic ash and portions of cal- 

 cite crystals. The plant remains present were too fragmentary 

 for determination. 



The remains of vertebrates are found among the upper grav- 

 els, on floors of tiny ravines, and embedded in clay or sand 

 layers. The content of the calcareous sand lenses (No. 9, Sec- 

 tion I), commonly a mixture of sand, lime, and clay, is in places 

 fully one half bone fragments. Bones are widely scattered 

 horizontally, and in a given locality may be found unrelated 

 parts of a skeleton mingled with bones of animals belonging to 

 entirely different species. No entire skeletons were found in 

 place, and it is probable that several feet of strata intervene 

 between portions of the same carcass. The species represented 

 by parts of skeletons are listed and described by JDr. Eaton 

 (pp. 144-151 following). Field evidence justifies the conclusion 

 that bones of animals which died on shore or floundered in bogs 

 or quicksand were redistributed by surface wash and running 

 water. 



