62 JOURNAL OF GEOLOGY— SUPPLEMENT 



introduction of lime and its subsequent removal as lime silicates 

 during the course of differentiation. The occurrence of cancrinite 

 and primary calcite is considered evidence of the introduction of CO2. 



The fact that limestone may, as a rule, be found in the same 

 general region as alkaline rocks cannot, in itself, be regarded as 

 having any significance, because limestone is of such extremely 

 widespread occurrence. Even in those cases which seem best to 

 indicate some connection between the limestone and the alkaline 

 rock, the application of the theory presents grave difficulties. In 

 the Haliburton-Bancroft area of Ontario, batholiths, dominantly 

 granitic, show nephelite syenites against bordering limestones. 

 The relation is striking, but since the theory demands the absorp- 

 tion of limestone by the magma, and the subsequent formation of 

 the alkaline rock by differentiation from the homogeneous magma, 

 the manner of differentiation should determine the location of the 

 alkaline rock, and the fact that this differentiate borders against 

 the limestone offers no real support to the theory. It is, moreover, 

 clear that the magma has not been desihcated for huge volumes of 

 it have crystallized to granite, unless the desilication was entirely 

 local, a possibility which does not seem to accord with the idea 

 that the limestone was absorbed as blocks which had sunk in the 

 magma to great depths. 



The writer has considered an alternative hypothesis which still 

 maintains some connection between the limestone and the alkaline 

 rock. The suggestion is that silica was subtracted from the magma 

 locally for the formation of amphibolites from the surrounding 

 limestone, a local desilication without important additions of lime 

 from the limestone.^ The occurrence of cancrinite could then be 

 considered as due to introduction of CO^ from the limestone, but 

 this supposition, like Daly's, does not offer an explanation for 

 the special abundance of sodaHte in these nephelite syenites. 

 There is no reason why an abundance of chlorine should be pro- 

 duced by interaction with limestone. 



On the other hand, if it is supposed that the nephehte syenite 

 belongs to the stage of great concentration of the volatile con- 



^ The quite similar suggestion recently oiiered by Foye appeared too late to 

 receive adequate recognition in this paper, Am. Jour. Sci. (4), XL (1915), 430. 



