DESCRIPTION OF FORMATIONS 469 



to experts for determination, but unfortunately they were lost before 

 being fully identified. Mr Hills regarded the abundant teeth found in 

 one stratum as belonging to a crocodile near Belodon prisons, and certain 

 fish remains as representing a ganoid similar to Catopterus gracilis. A 

 small gasteropod shell and eleven or twelve apparently determinable 

 species of plants were found. The Belodon and Catopterous remains 

 were found in the limestone conglomerate, occurring here about 50 feet 

 below the La Plata sandstone, and the fossil leaves in thin bedded, red- 

 dish, micaceous sandstones not far below the conglomerate. 



La Plata formation. — The strata of assumed Jurassic age occurring 

 below the Dakota Cretaceous in the Elk mountains of central western 

 Colorado were grouped together and called the Gunnison formation by 

 Eldridge in the Anthracite-Crested Butte folio. The formation is de- 

 scribed by Eldridge in these terms : 



"At its base is a heavy white quartzite, 50 to 100 feet thick, usually in a single 

 bed. Above it, in some cases succeeded by other sandstone layers, is a blue lime- 

 stone containing abundant fresh-water shells of the genera Limnea, Valvata, and 

 Cypris. The remainder of the formation consists of gray, drab, pink and purple 

 clays and marls, through which run thin intermittent beds of drab limestone." 



In the San Juan region and the adjacent country it is desirable to sub- 

 divide the Gunnison, grouping the two lower sandstones with the inter- 

 vening limestone and associated shales as the La Plata formation, and 

 the upper marls, clays, and sandstones as the McElmo formation. This 

 has been done in the Telluride and La Plata folios (3 and 4). 



The La Plata sandstones are commonly not indurated as in the Elk 

 mountains ; instead they are rather friable and crumbling, although of 

 homogeneous texture. Cross-bedding is a marked feature, and not infre- 

 quently a massive ledge as much as 100 feet in thickness has no promi- 

 nent division planes. 



Of the two sandstone members the lower is commonly thicker and 

 much more massive than the upper. The latter is in fact occasionally 

 thin bedded and shaly and may be inconspicuous. 



The calcareous member is very variable in character. On the San 

 Miguel river, in the Telluride quadrangle, it is in some places a pure 

 massive, blue-gray limestone in several beds and with almost no shale. 

 Usually dark calcareous and bituminous shales and thin bedded sand- 

 stones, with more or less of massive limestone, occur between the two 

 main sandstones and sometimes reach a thickness of nearly 100 feet. 



The total thickness of the La Plata formation varies, in the area we 

 have examined, from about 100 feet in the Ouray and Telluride quad- 

 rangles to 500 or more in the La Plata mountains, and it is known that 

 to the west all members increase still further in thickness. 



