R. A. Daly — Secondary Origin of Certain Granites. 209 



G. In each occurrence the gabbro contains xenoliths of the 

 more acid sedimentary rocks. These blocks are commonly 

 more or less digested and the product of this local solution is 

 always closely allied to, if not equivalent to, the granophyre- 

 granite phase. 



d. In each case there is correspondence though not equiva- 

 lence between the composition of the acid border-phase and 

 the average composition of the invaded formation. This 

 important fact is emphasized in Tables I, II, III, TV and V, 

 in which the silica and alkalies are either directly or inferen- 

 tially seen to be more or less abundant in the granophyre- 

 granite according to the relative abundance of those oxides in 

 the respective country-rocks. 



e. A considerable number of other examples not as yet 

 thoroughly studied have been noted in British Columbia and 

 Minnesota. The conditions are throughout identical or so 

 allied as to favor one explanation common to all the occur- 

 rences. 



f The assimilation theory is also supported by certain other 

 facts which have already been mentioned but merit a more 

 detailed discussion such as is attempted in the sequel. 



3. The principal objection to the doctrine of assimilation, 

 namely, the objection that chemical analyses disprove any genetic 

 relationship between intrusive and invaded formation at certain 

 accessible contacts, cannot hold, because that objection allows no 

 place for differentiation in the magma made compound by 

 assimilation. 



Asymmetry of the Intrusive Bodies. — There remains for 

 particular explanation the cardinal fact that all the intrusive 

 bodies are asymmetric. The granophy re-granite is always con- 

 centrated on one side of the intrusive, that is, along the upper 

 contact or the side away from which the enclosing sediments 

 dip. 



In all the localities the dips of the sedimentaries and of the 

 intrusive sheets are believed to have been flatter at the time of 

 the injection of the magma than those dips now are. It 

 is, indeed, possible that, in every instance, the gabbro sheet lay 

 practically horizontal during the period of cooling and consoli- 

 dation. In any case the granophyre-granites appears to have 

 always overlain their respective gabbroid associates. 



Three possible explanations have offered themselves for this 

 asymmetry, (a) It is conceivable that extensive assimilation 

 occurred only on the upper contacts ; or (h) the asymmetry 

 may be due to the density stratification of magma compounded 

 of gabbro and digested sediments ; or (c) due to a combination 

 of both those factors. 



One or more subordinate suppositions are necessary if the 

 assimilation be credited essentially to the upper contact. On 



