172 ROLLIN T. CHAMBERLIN 
exposition of the part block faulting has played in the extra- 
vasation of lava.‘ According to his belief, block faulting under 
tensile stress offers the principal outlets for the escape of 
lava. To quote: ‘‘The deepest fractures starting from the zone 
of potential magma should permit its eruption and intrusion 
between blocks that tend to part from one another by reason of 
the tensile stress.” The wide prevalence of normal faulting in 
mountains of the thick-shelled type should therefore be an impor- 
tant factor in the rise of magmas. ‘The steep inclination of normal 
fault planes carries these fractures to greater depths than the more 
gently inclined thrust faults. At the same time normal fault planes 
because of the governing tensile stress, at least locally, become the 
more ready avenues of escape for the lavas. While rhyolite and 
other acidic lavas have appeared in vast quantities in some places, 
andesitic and basaltic lavas appear to have been, on the whole, 
more abundant. ‘This may perhaps be in part because the greater 
liquidity of the basic lavas makes migration along narrow fissures 
easier for them than for the more viscous silicic magmas. 
SUMMARY 
The formation of thick-shelled mountains is characterized in 
general by much volcanic activity. There may also be important 
intrusives bearing a close relation to the mountain-making stresses. 
The growth of thin-shelled mountains, on the other hand, is 
accompanied by very little volcanic activity, at least within 
the truly mountainous belt. Little igneous activity of any sort 
is manifested in the marginal and most strongly overthrust por- 
tions of thin-shelled ranges; but in the heart of the deformed 
belts, where there has been more uplifting and the affected 
zone goes deeper, granitic and other intrusions are a common 
and probably characteristic feature. It is of course also to be 
recognized that a region which, in an earlier age, has undergone 
deformation of the thin-shelled type may, in a later age, after long 
continued denudation, participate in orogenic movements of the 
thick-shelled type and so become the scene of volcanic activity on 
a large scale. 
tJ. P. Iddings, The Problem of Vulcanism (‘Silliman Memorial Lectures”), 
Yale University Press (1914), pp. 79-81, 183-84. 
