Petrographical Relations of Laurentian Limestones. 33 



of the inclusions often hold a more acid feldspar, contain 

 no scapolite, and in place of the lime-rich augite, contain 

 more hornblende and some biotite. That is a vital 

 question, indeed, and unfortunately, it is a particularly 

 difficult one to solve, especially in the absence of chemical 

 data. 



Dr. Adams thinks there may be a transition in the 

 inclusions near the contact from the one composition to 

 the other, but if so, that is very abrupt. The occurrence 

 of labradorite in some of the most basic of these in- 

 clusions supports this idea. And at any rate, at least 

 four theories can be advanced to explain the differences 

 between them and the altered limestones ; they are : — 



1. That the same metamorphic forces, viz., heat, 

 pressure, emanations such as vapors and heated solutions, 

 etc., acted upon both classes of rocks, and that the. differing 

 results are due to the difference in degree and length of 

 time in which these forces acted. It seems obvious that 

 the action would be more intense and more prolonged in 

 the magma itself than about its edge. 



2. That the same forces acted possibly also, as in (1), 

 to a different decree and for a different time, but in the 

 case of the contact zone, in the presence of an excess of 

 limestone, and in the case of the inclusions, in the 

 presence of an excess of granite ; thus in the first case 

 the tendency would be for any minerals to become richer, 

 and richer in lime, while in the second case, the lime 

 which the minerals held would be more and more diluted, 

 so to speak, by accessions of more silica, alumina, iron, 

 alkalies, etc., from the granitic magma. 



3. That different forces acted on each class of rocks ; 

 that those of the contact zone were subjected to exhala- 

 tions and solutions from and accompanying the intrusion, 

 but that those of the inclusions were subjected to quite a 

 different influence — that due to the direct action of the 



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