168 ROLLIN T. CHAMBERLIN 
Cores of granodiorite and allied rocks having intrusive relations 
with the adjacent sedimentaries are very conspicuous ‘in the 
sharply compressed Sierra Nevada Range, the Mesozoic Cascades, 
the Coast Range of British Columbia, and various members of the 
Cordilleran chain extending into Alaska—a group folded, according 
to present information, at about the close of the Jurassic. Through- 
out this disturbed region the Jurasside batholiths are a dominant 
feature. | 
In some of these cases, at least, the intrusions seem to follow 
in consequence of the folding, and to appear beneath the most 
strongly arched portions of the ranges, presumably in consequence 
of reduced pressure. ‘That the arching process tends to relieve the 
pressure beneath and hence favors liquefaction and the penetration 
of magmas, is a principle long recognized and variously utilized | 
in explaining vulcanism.t But whether the arching and partial 
relief of pressure be a major or a minor factor in the actual lique- 
faction of the magma, the shearing movements involved in this 
type of deformation might well facilitate the transfer of magma, 
and would favor its insinuation near the surface, either as irregular 
batholithic bodies, or perhaps more likely as large lenticular or 
pancake-shaped intrusive masses whose thicknesses are much less 
than their horizontal extent. The general laccolithic shape, using 
the term in its broadest sense, would seem to be the favorite form.? 
Intrusions of this sort occur in both thin-shelled and thick-shelled 
ranges. 
THIN-SHELLED TYPE 
In those thin-shelled ranges in which overthrust faulting has 
been a dominant feature, intrusions formed in this way should not 
be conspicuous in the marginal portions where the phenomena of 
overthrusting are best displayed and the shell was thinnest, but 
rather in the heart of the deformed belt, where the shell involved in 
1 W. Hopkins, “Researches in Physical Geology,” Phil. Trans., Part I (1842), 
pp. 43-55; Eduard Suess, The Face of the Earth, Vol. I (1904), p. 170; W. H. Hobbs, 
“Some Considerations Concerning the Place and Origin of Lava Maculae,” Bevtrage 
zur Geophysik, Vol. XII (1913), pp. 329-61. 
2W. C. Broegger, Eruptivgesteine des Kristianiagebietes, IL (1895), pp. 116-53; 
Alfred Harker, The Natural History of Igneous Rocks (1909), pp. 60-87. 
