STRATIGRAPHY 565 



two or more feet across, although most of them are less than six inches. 

 The black shales' near the base of the formation have no interbedded 

 tillite, Interfingering with them are brown sandstones. In places the 

 shale-sandstone contacts are very uneven, but these suggestions of uncon- 

 formability are very restricted (see figure 6). 



The Bermejo formation is believed to be for the most part continental 

 in origin, although the few fragments of invertebrates observed may 

 indicate marine or brackish water conditions, The thickness and uni- 

 formity of the beds show that it was either laid down in a basin with 

 great marginal relief or else in a subsiding basin. The massive sand- 

 stones . are almost certainly water-laid. The clays undoubtedly are., 

 although they do not resemble marine beds and lack the fossils so com- 

 monly associated with clays deposited in the sea. The faceted pebbles 

 scattered through both shales and sandstones indicate the presence of 

 streams issuing from a glacier front or from glacier-capped highlands. 

 The black basal shales are thought to owe their color to original material 

 rather than to organic matter accumulated during their upbuilding. 

 The conclusion tentatively reached is that the Bermejo was laid down 

 in an interior basin, crossed by meandering rivers, with glacier-capped 

 lands on the margins. At times the glaciers extended well into the 

 basin, and then the coarse material which formed the tillites was 

 deposited either directly from the melting ice or by the streams close 

 to the edge of the ice. At other times the streams dropped their coarse 

 material closer to the margin of the basin, and only rock dust and fine 

 sand were carried far into the basin, where they were built into the 

 massive clays and heavy sandstones. It is very probable that the basin 

 was invaded by the sea from time to time, but the low temperatures 

 induced by the waters of the ice-fed rivers probably repelled all but a 

 few hardy forms of marine life, so that little record of them is to be 

 expected. 



CAJONE8 LIMESTONE 



The Cajones limestone lies immediately above the Bermejo formation 

 and is believed to be separated from it by a pronounced unconformity, 

 although it cannot be stated positively that such unconformable contacts 

 as were seen are not very restricted in lateral extent. 



No good exposure of this formation was seen on the traverse between 

 Cochabamba and Santa Cruz, although its outcrop was crossed, and 

 abundant float in the streams proved its presence. Subsequent work 



