562 N. L. BOW EN 



we expect precipitation of the micas in acid magmas, and formation 

 of orthopyroxene in such magmas is to be referred to other causes 

 than that proposed by Evans'^ and here adopted for basic rocks. 



When we turn to the case of carbonate rocks we find that the 

 reaction with granitic magma is of a different nature. It is often 

 observed that wall rock and inclusions of carbonates are altered to 

 silicate minerals.^ It has been assumed by some investigators, 

 therefore, that silica has been subtracted from the granitic liquid 

 and that this may occur to such an extent that some of the feldspar 

 molecules are transformed into the less siliceous, feldspathoid mole- 

 cules with consequent formation of alkaline rocks. This assumed 

 action is said by certain writers to be in agreement with Daly's 

 theory of the origin of alkaline rocks. We have seen above, how- 

 ever, that Daly assumes that superheated basaltic magma is the 

 starting point for all his syntectic magmas and that alkaline rocks 

 are differentiates of some of these syntectics, principally those 

 formed with carbonate rocks. We have pointed out on a preceding 

 page that, if adequately superheated basalt were available, it might 

 form, by solution of carbonates, a melilite basalt and, given the 

 latter, alkaline differentiates seem not impossible and so indeed 

 some alkaline rocks may be formed. Not all alkaline rocks can be 

 so explained, for much nephelite syenite shows intimate genetic 

 relations with granites and on Daly's general theory the original 

 basaltic magma would require to be silicated by solution of acid 

 material to form the granite and desilicated by solution of carbon- 

 ates to form the nephelite syenite. That the differentiates should 

 show evidence of both seems out of the question. The solution 

 of foreign matter must result in either desilication or silication 

 according to the preponderance of one or the other type of foreign 

 matter and subsequent differentiates should be in conformance 

 with one or the other but not both. 



This brings us back to the question whether a nephelite syenite, 

 intimately associated with a granite, could have been formed as a 

 result of desilication of the granite by carbonate inclusions, that is, 



' J. W. Evans, Qiiar. Jour. Gcol. Soc, Vol. XL VIII (1921), p. 133. 

 2 For example in the large-scale production of amphibolites in the Halibu'-ton- 

 Bancroft area. Adams and Barlow, Can. Gcol. Surv., Mem. No. 6 (1910). 



