LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 65 



ception of the alkaline rocks advanced in the present paper appears 

 to afford a rational explanation for this fact. It is considered that 

 the alkaline rocks are derivatives of the same magma as sub- 

 alkaline rocks, and belong to a late stage of concentration of the 

 volatile constituents of the magma. They are light differentiates 

 and therefore have a distinct tendency to occur as marginal and 

 satellitic bodies at a higher level than their sub-alkaline co-deriva- 

 tives. In those discrete mountain groups which lie east of the 

 Rocky Mountain front in Montana, erosion has, as a rule, pene- 

 trated only to a level where the igneous intrusives are dominantly 

 alkaline. Farther to the west, uplift and consequently depth of 

 erosion are relatively greater; the present surface has penetrated, 

 for the most part, below the level of the alkaline intrusives, and 

 the principal exposures are of sub-alkaline types. Farther to the 

 north, in the Rocky Mountain system in British Columbia, erosion 

 has as yet penetrated only one considerable igneous body, the Ice 

 River laccolith, and that, in conformity with the expectation of the 

 general tendency postulated, is an alkaline body.^ 



If the suggestion is correct, then, this striking division into 

 provinces is only in a remote manner connected with the tectonic 

 feature, the Rocky Mountain front. Relative depth of erosion is 

 accountable for the differences observed at the present surface. 



It appears, moreover, that the general tendency toward a 

 greater abundance of alkaline rocks in recent terranes as compared 

 with ancient terranes is assignable to an accompanying tendency 

 toward a greater depth of erosion in the ancient terranes. 



Smythe, attacking the subject from a somewhat different 

 viewpoint, has arrived at substantially the same conclusion as the 

 writer with regard to the importance of mineralizers in the forma- 

 tion of the alkaline rocks. ^ Daly, too, appears to consider it pos- 

 sible that the CO2 added when limestone is absorbed may be an 

 important factor and this may perhaps be true in some cases. The 

 writer would agree with Smythe in considering the production of 



' J. A. Allan, Geology of the Ice River District, British Columbia, abstract of thesis, 

 Massachusetts Institute of Technology, 191 2, and Geol. Survey Canada, Mem. §§, 

 1914, p. 209. 



=> C. H. Smythe, Jr., Am. Jour. Sci. (4), XXXVI (1913), 46. 



