210 jR. A. Daly — Secondary Origin of Certain Granites. 



the one hand, the invaded formations above and below the 

 gabbro might be lithologically so different that the one above 

 was much more subject to contact alteration than the one below. 

 This idea is at once declared irrelevant in the British Columbia 

 and Minnesota cases, where there is certainly no evidence of 

 differences of digestibility. 



On the other hand, it is possible that the original gabbro was 

 differently constituted, and thus more energetic in assimilation, 

 along the upper contact than elsewhere. Magmatic water or 

 other strong solvents may thus be conceived to have early con- 

 centrated in the upper zone of each sill. In favor of this view 

 would seem at first sight the fact that at Pigeon Point the zone 

 of external metamorphism is reported by Bayley to be much 

 wider on the upper contact than on the lower. (See Hg. 4.) 

 The same seems to be true in the Sudbury case, but is explained 

 by Coleman as noted on a previous page. The writer could 

 find no absolutely certain evidence of such differential meta- 

 morphism about the Movie sill, yet considers it as probable. 



That the conditions for the complete assimilation of the 

 invaded formations obtained throughout the intrusive bodies is 

 illustrated in the unmistakable digestion of xenoliths found at 

 all depths in the gabbro. As already pointed out, the assimi- 

 lation here belongs to the period immediately preceding the 

 consolidation of the gabbro. A much greater volume of sim- 

 ilar material derived from the interaction of gabbro and sedi- 

 mentary rock must have been formed from other blocks in the 

 hotter, more fluid, and more energetic magma of the preceding 

 period. There seems to be no possible doubt that most of that 

 material has diffused upward and now forms part of the grano- 

 phyre-granite zone. 



The simplest and most probable cause for that diffusion is, 

 as suggested in the field hypothesis for the Moyie sill, the dif- 

 ference of density between the acid magma of assimilation and 

 the enclosing gabbro. 



It is quite possible that the metamorphosing effect of the 

 new magma may have been greater than that of the original 

 pure gabbro. The new magma would presumably carry with 

 it the water derived from the digested sediments which, appar- 

 ently in every case, are notably more hydrous than the original 

 gabbro magma. The accompanying table shows the propor- 

 tion of water (or loss on ignition) found in the analyses of the 

 rocks of Pigeon Point. 



Eock. Per cent of water. 



Gabbro .- 1-04 



Intermediate rock 2*47 



Red soda granite ._ 1*21 



Slate . 3-24 



Quartzite (loss on ignition) 1*88 



