EVIDENCES OF THE OEEAT EEOSION. 
67 
ridges of granite, gneiss, and schist, traversing it at narrow intervals, and 
with their axes trending northwest and southeast. 
Here at last is the logical halting place. The sierra country beyond 
the Aubrey Cliffs was a mainland in Mesozoic time just like the Great 
Basin, and sent down its detritus into the Mesozoic sea, which washed its 
coast and stretched away eastward to join the Mesozoic ocean, which sub- 
merged the central portions of the North American continent. The Arizona 
coast trended northwestward. The Great Basin coast trended southwest- 
ward, and the two, if sufficiently prolonged, would meet somewhere in 
Southern Nevada. But of the localities near this imaginary junction we 
know almost nothing geologically. 
Reverting now to the distribution of the Mesozoic strata around the 
borders of the Grand Canon district, as explained in tlie preceding chap- 
ters, we find that they occupy the entire northern side and the entire 
eastern side and appear for a short distance upon the western side. The 
remaining portion of the periphery of the district is in greatest part and 
perhaps wholly a shore -line. Whatsoever extensions we assign to the 
edges of the Mesozoic, whether in the terraces or in the Echo Cliffs, are 
towards the original coast of the sea in which they were deposited ; and 
since we can find no reason for terminating them until we reach that coast, 
we seem compelled to infer that they once covered the entire district. 
The second case would raise the question whether these beds may not 
have thinned out to a merely nominal volume or vanished entirely in their 
extensions over the denuded area. The answer to this is strongly adverse. 
We have just noted that these extensions are towards the shore and not 
away from it, while the directions in which strata attenuate are usually the 
reverse. Towards the coasts they thicken. We have already noted how 
the strata in the terraces decrease in volume towards the east and southeast, 
and as we travel along the Paria Plateau and the foot of the Echo Cliffs, 
we find that they have reached a minimum. But all this I have allowed 
for in estimating the average denudation of the Grand Canon platform. If 
they are below the average volume in the Echo Cliffs, they are quite as 
much above it in the Valley of the Virgen. In connection with the possi- 
ble variations in the thickness it may be remarked that one of the most 
