VULCANISM AND MOUNTAIN-MAKING e/a 
The last thin-shelled range listed in the paper on the Colorado 
Rockies was the Serra do Espinhaco of Brazil. In the thrust- 
faulted portion of this ancient system very little igneous activity 
of any sort has occurred.*. Eastward for 170 miles toward the 
Atlantic Coast, in which strip the heart of this mountain system 
presumably lay, there remains today only what has been called 
the Archean Complex. This region is characterized by many 
massive intrusions. Some of these may possibly have been 
injected at the time of the Serra do Espinhacgo orogeny, though 
there is no evidence as yet bearing on this question. . 
_ Similarly in the Canadian Rockies very little igneous activity 
occurred in the overthrust region of Alberta; but farther west some 
of the massive intrusions in British Columbia may prove to have 
been related to this thrusting. 
The outer marginal portions of ranges of this type, both folded 
and faulted, are particularly superficial. In the great over- 
thrusts but very shallow flakes have been moved. ‘The very low 
inclination of the fault planes does not carry them to great depths. 
Beneath the planes of overthrusting, the underlying strata, if of 
incompetent material, are found to be contorted in many places, 
but this folding rapidly dies out away from the thrust planes. 
Such shallow deformation does not greatly facilitate the movement 
of magmas. But back in the heart of the deformed belt the dis- 
turbance goes much deeper, and uplifting with relief of pressure 
beneath is more pronounced. Here, as the above-mentioned illus- 
trations seem to show, intrusions tend to develop. 
THICK-SHELLED TYPE 
As pointed out, the thick-shelled mountains have been char- 
acterized by open, gentle folding, moderate crustal shortening 
affecting a deeper zone, by strong uplifting, and the extravasa- 
tion of much lava.? Vertical diastrophism seems to dominate 
over horizontal. Normal faulting is an important accompani- 
ment, occurring either incidentally as a part of the uplifting 
process or as a result of subsequent relaxational movements 
of the raised plateau-like area. Iddings has given an excellent 
tE. C. Harder and R. T. Chamberlin, ‘‘The Geology of Central Minas Geraes, 
Brazil,” Jour. Geol., Vol. XXIII (1915), pp. 341-78. 
.? Jour. Geol., Vol. XXVII (1919), p. 251. 
