THE PEEMIAF TEEEACE. 
43 
the eroded edges of the Mesozoic system and the distribution of the outliers 
point strongly to the conclusion that all of its formations once stretched 
unbroken across the whole Carboniferous platform which drains into the 
Grand and Marble canons, and that during Tertiary time they have been 
removed from it by erosion. The dimensions of this platform are indeed 
vast, being in round numbers about 13,000 or 14,000 square miles. Before 
accepting the conclusion that a denudation so extensive has really occurred, 
the geologist needs to be assured that the Mesozoic strata were really 
deposited over the whole area. The deposits of that age in other parts of 
the world are often of a very local character. Their continuity is fre- 
quently broken by the intervention of land areas; their bulk varies much 
from place to place; members which have great volume in one locality 
have only a nominal one in another, or perhaps have no assignable counter- 
part or equivalent whatever in other regions. May it not have been so 
here, and may not the great denudation be too hasty and too large a con- 
clusion from imperfect premises'? The answer to this question will depend 
largely upon the facts disclosed in the interior of the Grand Canon district, 
and upon facts of structure which will be treated of in other parts of this work. 
THE PEE MIAN. 
The idea of a terrace is not so typically represented in the Permian as 
it is in the superior formations. In many parts of the great stairway it 
clearly forms the lowest step; in others it forms one cliff with the Trias; in 
still others it is beveled off and covered with alluvium. On the whole it is 
more frequently presented as a distinct terrace. There is another qualifica- 
tion which requires some mention, because when we refer to the geological 
map to study the surface distribution of the strata we should find some 
anomalies unless the point referred to were duly explained. 
Wherever we encounter a cliff which discloses the upper Permian 
beds we find at the summit of the escarpment a band of pale-brown sand- 
stone of very coarse texture, often becoming a conglomerate. Its thickness 
is usually from 40 to 75 feet In a few places it is wanting from its projjer 
