646 WHITMAN CROSS 
Newberry’s descriptions and sketches show how strong these charac- 
ters are in the greatly dissected country adjacent to Grand River. 
The base of the La Plata sandstone is probably to be taken as 
at the horizon where the light-colored, cross-bedded, massive strata 
give way to dark red sandstones of the Trias. The unconformity or 
stratigraphic break below the La Plata, discussed in the next section, 
is not always in evidence in the Plateau country and indeed there 
is in many places as near an approach to perfect conformity as is | 
commonly found within a given formation between sandstones of 
different textures. 
On the northern side of Cafion Colorado, near its head, the La 
Plata rests on thin-bedded reddish sandstones of fine grain near the 
top of which are calcareous layers. Some of the beds contain small 
chert fragments. In Fig. 6 is shown the nearly white, cavernous, 
cross-bedded strata, at the base of the La Plata, immediately beneath 
which are the strongly calcareous sandstones of dark-red color which 
on weathering yield large nodular masses, such as those in foreground. 
This is believed to be the line between the Jurassic and Triassic beds 
at this point. 
The continuity of the La Plata sandstone from the San Juan 
Mountain flanks down the Dolores and San Miguel valleys, around 
the La Sal Mountains to Grand River Valley, may be said to be 
perfectly plain and incontestible. In this distance the most notable 
change in the formation is its increased thickness. The massive 
texture and even grain of many strata, cross-bedding, variation in 
color, and other marked features are but emphasized by the greater, 
volume. The intermediate strata are most variable in character, 
yet everywhere the two great massive sandstone members are separated 
by beds distinguishable through their thin bedding, darker color, 
and richness in calcareous cement or development of limestone. 
From the district covered by our reconnaissance the ledges of 
gray, pink, or orange La Plata sandstone can be seen stretching to 
the west and south into the belt traversed by Green and Colorado 
Rivers, where Powell has described the White Cliff sandstone. This 
great unit in the Plateau country section was never described accurately 
nor in detail for any given locality, but there seems to be no ground 
for questioning the assertion of Powell that this formation is continu- 
