252 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



main mass of this intrusive is olivine diabase. Towards its upper contact the 

 diabase appears to grade into a pink rock, which proved to be a hornblende 

 syenite. The syenite makes a sharp contact with the argillites at the roof 

 of the sill. The floor rock is also an argillite. The feeding channel or channels 

 must have traversed the calcareous shale, limestone, and jasper of the lower 

 TJnkar series. Noble does not discuss the genesis of the syenite. ' Is this rock 

 not a gravity differentiate from a syntectic of the various sediments dissolved 

 in diabase magma? Since the argillites are dominant, one must expect for 

 the differentiate a degree of acidity intermediate between that of the lighter 

 differentiate in a Moyie sill which has assimilated quartzite, and that of the 

 lighter differentiate in a sill which assimilated limestone. This expectation 

 seems to be matched by fact; 



In these and other cases which might be cited, the chemical composition 

 of each lighter, salic pole in differentiated sills varies with the chemical com- 

 position of the invaded sediments. Herein rests a powerful argument favour- 

 ing the secondary origin of the respective salic or alkalic magmas. As indicated 

 in each instance, the intrusive mass is stratified in' the way demanded by the 

 theory of gravitative differentiation in a syntectic. 



GENERAL CONCLUSION AND APPLICATION. 



The remaining paragraphs of this chapter are devoted to the broader bearing 

 of the main conclusions regarding the Moyie sills. The statement is almost 

 identical with that already published in the writer's 1905 and 1906 papers, but 

 a few changes have been made in the form of presentation. 



Sooner or later experience must teach every careful field student of igneous 

 rocks the truth of the principle of magmatic differentiation. That principle 

 is, indeed, so generally accepted by petrologists that it may be considered as a 

 permanent acquisition in the theory of their science. Yet it is a long step 

 from, the recognition of the doctrine to its application to the origin of igneous 

 rocks as actually found in the earth's crust. The principle becomes really 

 fruitful, in fact becomes first completely understood and realized, when certain 

 chief problems have been solved. 



Among those problems there are naturally three that are fundamental. 

 Only after they are solved has petrology done that which it has set out to 

 do, namely, determine, under the difficult conditions of earth study, the true 

 nature and genesis of rocks. The first insistent question is, in every case, what 

 was the magmatic mixture or matrix from which the material of the existing 

 rock-mass or rock-masses was produced through differentiation? The second 

 question is, how far did the differentiating process operate ? The third insistent 

 question is, what was the process of differentiation itself? 



All three problems are interdependent and involve a study in structural 

 geology. They cannot be solved simply by acquiring even the fullest informa- 

 tion to be derived from single plutonic contacts, nor, as a rule, from such as 

 may be derived from entire ground-plan contact lines. On the other hand, 



