642 WHITMAN CROSS 
a pure massive blue-gray limestone in several beds and with almost 
no shale. Usually dark calcareous and bituminous shales and thin- 
bedded sandstones, 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 roo feet in the Ouray and Telluride 
quadrangles to 500 or more in the La Plata mountains, and it is 
known that to the west all members increase still further in thickness. 
The sandstones are almost wholly quartzose, and their normal 
color adjacent to the San Juan Mountains is white or gray; but 
yellow, orange, and red tints have been observed in that region. The 
cement is often calcite. 
In the Red Beds paper (8) were given the observations of Spencer, 
who traced the La Plata sandstones to Paradox and Sindbad valleys, 
west of the Dolores, and of Gane, who followed them down the San 
Juan Valley to the Colorado Canyon. Both noted the prevalence 
of orange or pink color in the lower country. 
On the strength of these observations and a study of literature it 
was concluded that “the La Plata Formation is seemingly equivalent 
to the White Cliff sandstone. Its local assumption of red color has 
led to confusion with the Vermilion Cliff in certain districts and a 
reference to the Trias.’’ This correlation is considered to be amply 
substantiated by the recent observations. As this matter is of much 
importance to an understanding of Plateau geology the grounds for 
this correlation will be given in some detail. 
The first point to be considered will be the relation of the La 
Plata sandstones of the San Juan slopes to the ‘“‘Triassic” of Peale 
in the Uncompahgre Plateau and lower Dolores Valley. The con- 
tinuity of the La Plata exposures from the Telluride area northwesterly 
through the San Miguel Canyon and Plateau to the slopes of the 
higher Uncompahgre Plateau leaves no room for doubt that the upper 
part of Peale’s “Triassic” consists of the La Plata. Moreover, 
a careful examination of Peale’s descriptions of the ‘“‘ Triassic”? sand- 
stone shows that characterization to be fully in harmony with this 
idea. He repeatedly emphasizes a distinction between the upper, 
light-colored and the lower dark-red sandstones. ‘To illustrate this 
