VULCANISM AND. MOUNTAIN-MAKING 167 
thicker-shell type of mountains in which vertical movements are 
more pronounced and horizontal thrusting and shortening less 
couspicuous, the Tertiary Cascades of the Pacific Coast, the Western 
Andes, and the Abyssinian Mountains were cited. In contrast with 
the preceding, the growth of these ranges was marked by the 
extravasation of vast floods of lava. 
In this treatment of the subject it was not explicitly stated that 
extrusive vulcanism alone was considered, although the text 
would seem to the writer to convey that idea clearly enough. In 
view of the fact that intrusions did not enter vitally into the 
Appalachian section of Pennsylvania or that of the Colorado 
Rockies,‘ this phase of vulcanism had no place in those studies, and 
so the topic of intrusions was not introduced into the comparison of 
thin-shelled and thick-shelled ranges in general. But there are, 
however, certain cases in which plutonic rocks appear in ranges of 
the thin-shelled type in such a way as to suggest that intrusions 
on a large scale may be a common habit of this type. It is because 
of the feeling that the absence of any statement covering the 
intrusive phase of vulcanism might convey erroneous ideas, and 
perhaps lead to more or less justified criticism, that the present note 
is added. 
Many folded mountain ranges of both thin-shelled and thick- 
shelled types are characterized by cores of crystalline rock, in 
considerable part of igneous origin. In many of these the crys- 
talline rock clearly belongs to an old terrane arched up in the 
folding process and exposed by erosion; but in many other cases 
the intrusive relations of the igneous rock lead to the belief that it 
was intruded into the axis of the folded range in a late stage of 
the arching process. The wide prevalence of this phenomenon has 
been emphasized by Daly in the following terms: 
Granitic intrusion of the batholithic order, to observed levels, always 
follows periods of the more intense orogenic movement. This implies that 
the greatest abyssal injections of the earth’s crust by magma are genetically 
associated with the horizontal shearing of a superficial earth-shell which is 
much thinner than the whole crust.? 
* Except in the pre-Cambrian complex, which has nothing to do with the Rocky 
Mountain diastrophism. 
2 Reginald A. Daly, “Geology of the North American Cordillera at the Forty- 
ninth Parallel,” Geol. Surv. of Canada, Mem. 38, Part II (1912), p. 573- 
