TRIASSIC FORMATIONS. a9 
conglomeratic, thus showing a still closer relation to the main land mass. 
The similarity of the beds at the top and the bottom of the Maroon series 
indicates the duration of similar conditions of deposition for a long period 
of time, although the prevailmg coarseness of the sediments indicates rapid 
erosion and sedimentation. To account for such prolonged rapidity of 
erosion and such similarity of deposital conditions, we may suppose a land 
area which was very mountainous and a gradually sinking shore line, the 
subsidence of which kept pace with the building up of the beds. The 
change from the coarse and varied Maroon beds to the more uniform red 
sandstones of the Triassic shows a slight though well-marked change. As 
in the Maroon beds, however, the continued uniformity of the Triassic 
sandstones, which are similar from top to bottom, shows the continuation 
of the gradual subsidence which has been noted for the underlying beds. 
The greater fineness of grain in the detrital material, however, and 
the greater purity (for the Triassic sandstones consist mostly of quartz 
sorted by wave action), indicate a less but still noteworthy amount of 
erosion, a change which may be explained by the gradual degradation 
of the land. The encroachment of the sea upon the land, which has been 
inferred from the lithological composition, is proved by the overlapping of 
the red Triassic sandstones upon the granite in a large part of Colorado.' 
GUNNISON FORMATION. 
Above the deep-red sandstones comes another series of beds with dis- 
tinct and persistent lithological characters. This series consists of a gray 
or yellow basal sandstone, often calcareous, overlain by reddish, grayish, or 
_variegated shaly sandstones. The thickness of the sandstone is about 50 
to 75 feet, and that of the shales above averages perhaps 225 or 250 feet. 
These beds are well exposed on the side of Red Butte, where they are 
inverted; and also on the west side of Maroon Creek, where they are in 
their normal position. 
On Red Butte much of the variegated appearance of the beds has been 
found to be due to a bleaching process analogous to that which has already 
been mentioned. The normal color of the beds appears to be a red brown, 
a little lighter than the typical color of the Maroon formation. There are 
‘Hayden, Ann. Rept. U. 8. Geol. and Geog. Sury. Terr., 1874, p. 44; A. R. Marvine, Ann. 
Rept. U. 8S. Geol. and Geog. Sury. Terr., 1873, p. 142. 
