298 7?. A. Daly — Mechanics of Igneous Intrusion. 



necessary to compare as well the effect of the rising of the 

 isogeotherms in fusing stratified formations. This paper has 

 already assumed such proportions that that subject may well 

 be left in abeyance. Suffice it only to point out that the 

 igneous rocks of most intrusive bodies are demonstrably exotic 

 and have penetrated considerable distances vertically into their 

 invaded formations which are not fused because of the rising 

 of the isogeotherms. The fusion of rocks by this method can- 

 not, therefore, of itself explain the formation of the actual 

 chambers opened to human sight by secular denudation. 



One must feel a certain hesitancy in taking a definite posi- 

 tion on a matter of such fundamental importance ; yet a cate- 

 gorical statement may bring into sharper relief the main 

 conclusions to which the writer has come. Dikes, sheets, lac- 

 coliths, " bysmaliths," and perhaps a few of the smaller 

 stock-like, plutonic bodies are conceived to be due to crustal 

 displacement permitting intrusion ; in the preparation of the 

 greater and much more important subterranean magma cham- 

 bers, marginal assimilation is believed to be a true cause, but, 

 in the large, to be quite subordinate to magmatic overhead 

 stoping, while bodily crustal displacement is in but indirect 

 control inasmuch as it only localizes the areas where stoping is 

 to form the chambers ; and abyssal assimilation of stoped-out 

 blocks, supplemented by the subordinate marginal assimilation, 

 may be held responsible for the preparation or notable modifi- 

 cation of magmas, whence come, through differentiation, most 

 of the igneous rocks of the globe. The plateau-basalts would 

 appear to represent the one widely distributed kind of magma 

 not essentially affected by assimilation. 



Geological Survey of Canada, Ottawa, Canada. 



