VULCANISM AND MOUNTAIN-MAKING: 
A SUPPLEMENTARY NOTE 
ROLLIN T. CHAMBERLIN 
University of Chicago 
In a recent paper on “The Building of the Colorado Rockies,” 
the writer compared and contrasted the Rocky Mountains of 
Colorado, an example of a thick-shelled range, with the Pennsyl- 
vania Appalachians, which belong to the thin-shelled type. One 
of the striking differences developed was that the building of the 
more deeply rooted Colorado mountains was accompanied by the 
extrusion of much lava, while the shallower Appalachians were 
formed with the extrusion of but little lava. A further comparison 
was made between different portions of the Rocky Mountain 
chain itself. In Colorado, where the deformed zone extended deep 
below the surface and the vertical element in the deformation was 
distinctly large, lava flows appeared in abundance; while in the 
Canadian Rockies, where the deformed zone was much shallower 
and suffered intense horizontal thrust, but very little volcanic 
activity occurred. 
The study was really concerned with only these ranges, but 
some of the ideas developed were extended, as tentative suggestions, 
to other mountain systems. The Alps, the Scandinavian chain, 
the Scottish Highlands, and the Serra do Espinhag¢o of Brazil were 
cited as probably belonging to the thin-shelled type because of their 
sharp folding, extensive thrust faulting, and great crustal shorten- 
ing, and it was stated that ‘‘only moderate igneous activity” 
was associated with their development.? As representatives of the 
* Jour. Geol., Vol. XXVII (1919), pp. 248-51. 
2 This refers only to the strongly deformed portions of these ranges, since any 
igneous manifestations outside of these belts would also be outside the thin-shelled 
tracts, even though they be more or less related genetically to the mountain-making 
stresses. In the case of the Alps, the statement refers of course to the intense Tertiary 
diastrophism. Whatever relation the granites in the axial portions of the Alpine 
ranges may have had to the Hercynian and earlier orogenic movements is not here 
considered. 
166 
