1890.] Geology and Paleontology. 469 
working hypothesis that the measure of the strength of the crust is a 
prominence or a concavity about 600 cubic miles in volume. 
If this hypothesis is strictly true, then there should be no single 
mountain mass and no single valley, due purely to the local addition 
or subtraction of material, having a greater volume than 600 cubic 
miles. At least four kinds of mountains and valleys are due simply to 
the addition and subtraction of material: (1) mountains of extravasa- 
tion (such as volcanic cones), beneath which the pre-existent terranes 
lie undisturbed ; (2) mountains of circumdenudation, produced by the 
removal of surrounding material ; (3) mountains produced by extrava- 
sation avd circumdenudation ; (4) valleys of erosion, unaccompanied 
by phenomena of displacement. 
A large number of such mountains and valleys exist, and some of the 
largest occurring in the United States have been mapped in contours 
by the U. S. Geological Survey, so that their volumes can be com- 
puted readily. 
San Francisco Mt., in Arizona, a result of extravasation, has a 
volume of 40 cubic miles 
Mt. Shasta, probably due to extravasation only, has a volume of 80 
cubic miles. 
The Tavaputs Plateau, or Roan Mt., lying on the borders of Utah 
and Colorado, and produced by crepdildeniidlation has a volume of 
700 cies iles 
aylor, and the Taylor Plateau, in New Mexico, resulting from 
aaia and circumdenudation, have jointly a volume of 190 
cubic miles. 
The Henry Mts., resulting from volcanic intrusion and circumdenu- 
dation, have a volume of 230 cubic miles. 
The Sierra La Sal, a mountain group of the same type, has a vol- 
ume of 250 cubic miles. 
The deeper portion of the Grand Cafion from the Colorado, from 
the mouth of the Little Colorado to the mouth of Kanab Creek, is 
due to the removal of 350 cubic miles of rock. 
The Tavaputs Plateau slightly exceeds the hypothetic limit; the 
other illustrations fall within it, 
In view of the phenomena cited, and of the considerations and 
comparisons adduced, it is believed that the following theorem or 
working hypothesis is worthy of consideration and of comparison with 
additional facts: Mountains, mountain ranges, and valleys of magnitude 
equivalent to mountains, exist generally in virtue of the rigidity of the 
earth s crust; continents, continental plateaus, and oceanic basins exist 
