REPORT OF TEE CHIEF ASTRONOMER 779 



SESSIONAL PAPER No. 25a 



6. Magmatic differentiates of syntectics of Class B. 



Representative rocks: abnormal granites (Moyie sills, Sudbury sheet, 

 etc.); most nephelite and leucite rocks; some corundum-bearing 

 types; essexite, etc. 



7. Magmatic differentiates of syntectics of Class C. 



Representative rocJcs: granodiorite, dacite, some syenites, etc. 

 S. Hybrid magmas formed by mixtures of two or more of the above-mentioned 

 nine types. (?) 

 Representative rocks : ( ?) 

 9. Transition magmas marking incomplete differentiation. 



Representative rocks : ' intermediate ' rocks of Moyie sills, Pigeon Point 

 intrusive, Sudbury sheet, etc.; transition types in differentiated 

 dikes, laccoliths, etc. 



Application of the Theory to the Forty-ninth Parallel Rocks. 



Introduction. — The assembling of old and new features in the general theory 

 has been the product of the years of active field work on the Boundary section. 

 It represents an attempt to find explanation for a multitude of new facts ob- 

 tained during ten field seasons. At many points the reader has seen that this 

 theory has already been tested for such bodies as the Okanagan composite 

 batholith, the Purcell sills, the Bayonne batholith, and the stocks of the Selkirk 

 range. Needless to say, the theory has not been brought to the present shape 

 without abundant reference to igneous fields elsewhere. Neither the personal 

 observations in the field, nor those described by other writers, have discovered 

 facts which are irreconcilable with this general theory. Its strength is obviously 

 due to its being a synthesis of many ideas from the leaders of penological 

 thought for the last two generations. The writer's principal contribution to 

 it has been the negative one of showing a stumbling-block which has stood in 

 the way of advance in theory. The leaders in modern petrology, for the most 

 part, have denied the efficiency of magmatic assimilation because they have 

 generally failed to find hybrid rocks at molar and xenolith contacts. The 

 explanation of this patent fact is found in the stoping hypothesis. That hypo- 

 thesis demands that hybrid rocks or direct evidence of assimilation shall normally 

 fail at visible batholithic contacts. In making this failure an objection to the 

 stoping hypothesis several writers have shown that they did not understand it 

 fully. If stoping be accepted, abyssal assimilation on the large scale must be 

 accepted, and mere differentiation of original magmas should no longer hold its 

 entirely dominating place in petrogenic theory. 



The explanation of facts which are intended to be covered by a theory do 

 not suffice to prove that theory. It should do that as a matter of course. To 

 be final it should take care of all new facts as they are discovered, and it should 

 be prophetic of new findings in nature. The writer does not hold that the 

 outlined theory has been sufficiently tested to be regarded as final. On the 

 other hand, its ability to explain the hundreds of igneous bodies which occur 



