RECONNAISSANCE IN COLORADO AND UTAH 641 
and Gunnison both antedate Como by several years, if I am cor- 
rect in thinking that the first definite proposition to use Como as 
a formation name was made by Scott (35, p. 477) in 1897.! 
Beneath the vertebrate-bearing fresh water Jura are 100 to 120 
feet of “bluish and grayish gypsum-bearing clays in which thin 
layers of fine-grained sandstone and nodular ledges of limestone are 
interspersed.” ‘These Riggs refers to the ‘‘marine Jura,” although 
they are destitute of fossils. This assignment must be considered 
questionable in view of the fact that no marine Jurassic beds are 
known in Colorado except in the northwestern part where they 
apparently are to be correlated with occurrences in Utah and Wyom- 
ing. The few hundred feet of reddish sandstones below the ‘‘marine 
Jura” are referred by Riggs to the Trias, in accordance with Peale’s 
view. This assignment will be discussed in the next section. The 
underlying pre-Cambrian granite is spoken of (perhaps inadvertently) 
as “intrusive” in the sandstones. 
LA PLATA FORMATION 
The term La Plata formation has been applied in the San Juan 
folios and other publications to the lower part of what Eldridge 
described as the Gunnison formation (13). 
The La Plata consists of two massive sandstone members with an 
intermediate member of more thinly-bedded sandstones and a vari- 
able amount of bluish freshwater limestone. The 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 infrequently a massive 
ledge as much as 100 feet in thickness has no prominent 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 
t The reference by Knight to the Como beds as marine (American Journal of 
Science, 3d Series, Vol. V, 1898, p. 380) is clearly a mistake as he later names the 
marine Jura of Wyoming the Shirley beds, reserving the term Como beds for the 
jreshwater strata above the Shirley (Bulletin of the Geological Society of America, 
Vol. XI, 1900, p. 377). 
