214 R. A . Daly — Secondary Origin of Certain Granites. 



4. Mere differentiation of an original magma (through 

 density stratification) cannot readily explain the slight but 

 certain excess of silica along the lower contact of the Moyie 

 sill. That degree of acidification is readily understood on the 

 assimilation theory. 



5. Along the British Columbia boundary a large number of 

 contemporaneous gabbro sills of practically identical min- 

 eralogical and chemical composition have been found. In 

 most of these no true granite-gran ophy re zone occurs. The 

 composition of these gabbros is essentially equivalent to that 

 of the gabbro in the central part of the Moyie sill ; yet, on the 

 pure-differentiation theory we should expect a distinct differ- 

 ence of composition between these other sills and the basic 

 pole of differentiation in the Moyie sill. The assimilation- 

 differentiation theory finds no difficulty in the essential equiva- 

 lence of composition. 



6. The assimilation-differentiation theory demands that a 

 great absolute amount of thermal energy be credited to a sill 

 in which secondary granite has been formed ; that sill must 

 always be thick. Other things being equal, granite formed by 

 mere differentiation from an original magma should be found 

 also in sills of less thickness, though here again the absolute 

 thickness must be considerable. True granite with the rela- 

 tions described in this paper has never been found as a contin- 

 uous zone in any intrusive sheet 500 feet or less in thickness. 

 On the pure-differentiation theory it is difficult to understand 

 why differentiation should afford true granite in a sheet of the 

 strength observed at Pigeon Point, and should not afford a 

 true granite zone in a sheet 400 or 500 feet thick. The 

 assimilation-differentiation theory readily interprets the fact as 

 due to the relatively enormous amount of heat required for 

 the generation of the granite-gran ophy re zone, namely, an 

 amount of heat characteristic only of thick intrusive sheets. 



7. The pure-differentiation theory has to face another diffi- 

 cult question which does not arise if the assimilation-differen- 

 tiation theory be accepted. Why was differentiation in the 

 original magma postponed to the moment of intrusion ? This 

 difficulty is, of course, by no means conclusive against the 

 pure-differentiation theory, but it means one more unavoidable 

 theoretical burden weighting the pure-differentiation theory in 

 a way which renders, by contrast, the assimilation-differentia- 

 tion theory one of relative simplicity and, by so much, of 

 greater strength. 



General Application. — In the foregoing discussion the sec- 

 ondary origin of some granites has been deduced from the 

 study of intrusive sills or sheets ; but it is evidently by no 

 means necessary that the igneous rock body should have the 



