250 ROLLIN T. CHAMBERLIN 



breaking out long after the Appalachian folding, they appear to 

 be genetically related to faulting nearer the coast in a diastrophic 

 movement which occurred toward the close of the Triassic. Lavas 

 and tuffs caused no trouble in measuring the Pennsylvania moun- 

 tainous section. 



In view of the fact that the Pennsylvania Appalachians, which 

 show less volcanic activity, have resulted from the crumpling of a 

 comparatively thin surface shell, while the Colorado Rockies, with 

 more extensive vulcanism, involved a thicker shell, it is suggested 

 that there probably is a genetic relation between the depth to which 

 the roots of a mountain range extend and the amount of vulcanism 

 associated with it. It seems very rational- a priori that if the zone 

 disturbed in the formation of a folded range does not extend far 

 below the surface, it would not be likely to lead to the generation of 

 magmas nor greatly facilitate their escape if already present in the 

 undisturbed depths below, even though the folded zone itself may 

 be intensely deformed and shear strongly over the underlying base. 

 But if, on the other hand, the disturbed zone reaches down to 

 considerably greater depths, even though the deformation itself be 

 not so intense, it may readily afford better opportunities for the 

 generation or the rise of magma to the surface. At many points 

 the difference in depth of disturbance may be sufficient to decide 

 between a rise of magma and no upward movement of the magma, 

 or by operating through relief of pressure, perhaps decide between 

 potential magma and actual magma. 



On theoretical grounds the thin-shell mountains should be 

 accompanied in their growth by relatively little vulcanism, while 

 the growth of thick-shell mountains should be attended by relatively 

 greater outpourings of lava. Looking over the mountain systems of 

 the globe one might select, as some of the conspicuous illustrations 

 of sharp folding, great thrust faulting, and much crustal short- 

 ening, the Alps, the Scandinavian chain, the Scottish High- 

 lands, the Backbone Range of Brazil, and perhaps the Rocky 

 Mountains of Alberta. These would seem to be mountains of the 

 thin-shell type as nearly as can be judged without actual measure- 

 ments upon them. In each of these cases it will be observed that 

 only moderate igneous activity was associated with the mountain 



