GREATER PROBLEMS OF PHYSICAL GEOLOGY. 
63 
not with the idea of throwing light upon it, but to guard 
against a misapprehension which would otherwise be sure 
to occur. 
Geologic history discloses the fact that some great areas 
of the earth’s surface which were in former ages below sea- 
level are now thousands of feet above it. It also gives us 
reason to believe that other areas now submerged were in 
other ages terra firma. Our western mountain region at the 
beginning of cenozoic time was at sea level. It is now, on 
an average, 6000 feet above it. The great Himalayan plateau 
contains early cenozoic beds full of marine fossils which now 
lie at altitudes of 14000 feet or more. The whole North 
American Continent has, since the close of the paleozoic, 
gained in altitude. Now, it is sufficiently obvious that the 
theory of isostasy offers no explanation of these permanent 
changes of level. On the contrary, the very idea of isostasy 
means the conservation of profiles against lowering by denu¬ 
dation on the land and by deposition on the sea bottom, pro¬ 
vided no other cause intervenes to change those levels. If, 
then, that theory be true, we must look for some independent 
principle of causation which can gradually and permanently 
change the profiles of the land and sea bottom. And I hold 
this cause to be an independent one. It has been much the 
habit for geologists to attempt to explain the progressive 
elevation of plateaus and mountain platforms, and also the 
foldings of the strata by one and the same process. I hold 
the two processes to be distinct and having no necessary re¬ 
lation to each other. There are plicated regions which are 
little or not at all elevated, and there are elevated regions 
which are not plicated. Plication may go on with little or no 
elevation in one geologic age and the same region may be 
elevated without much additional plication in a subsequent 
age. This is in a large measure true of the Sierra Nevada 
platform, which was intensely plicated during the paleozoic 
and early mesozoic, but which received its present altitude 
in the late cenozoic. 
Whatever may have been the cause of these great regional 
