60 
OROGENY AND EARTH’S ROTATION 
that of the other. In direct opposition stands isostasy which 
premises mountain-forming where the thickest sections of rock 
have been removed. 
Although rock-expansion through rise in thermal increment 
appears not to be an initial factor in orographic genesis it prob¬ 
ably has an appreciable, and perhaps determining, effect upon the 
locus of volcanic activity, mainly through supplying that extrusive 
force which is oftenest ascribed to hydrostatic pressure. To what 
extent it directly contributes to the extravisation of lavas is not 
yet very clear. No doubt there are a number of distinct physi¬ 
cal forces at work. Operating separately they give expression to 
quite different phenomena. Acting in conjunction they result in 
vulcanism. 
Regional rock-expansion would, to be sure, locally intensify 
rock-compression and would express itself in a way akin to hy¬ 
drostatic pressure. For the latter is often mistaken, in all like¬ 
lihood, for this very phenomenon. But rock-expansion of the 
sort that Reade argues for seems much too slow in its action to be 
of any material assistance as furnishing the chief pressure for the 
outwelling of lavas. Local stresses due to orographic folding, 
whatever may be the source, appear to have far greater mechanical 
efficiency, and are, moreover, not only commensurate with the 
intensity of the extravasation, but the two keep pace with each 
other. 
Perhaps if we neglect the factor of rock-expansion altogether 
in problems of tectonics the ultimate valuations of the several 
forces wdll not be appreciably affected. 
The sharp, unfamiliar profiles of the desert ranges, as they ap¬ 
pear to the traveler fresh from pluvial climes, wherein many of 
them seem to have been bodily upraised through profound fault¬ 
ing and displacement in such a manner that the tumbled moun¬ 
tain blocks have been aptly likened to floating ice-cakes in a river 
gives rise to impression that orographic movement is mainly ver¬ 
tical rather than horizontal in character. As such it seems to 
stand as a type by itself. In this particular this hypothesis of 
mountain-building is so diametrically opposed to the contractional 
theory that it serves as a direct challenge to the latter’s verity. 
In its main features Dutton’s theory of isostasy is, as Palache 
observes, an hypothesis of conservation of equilibrium in the 
