282 Canadian Record of Science. 



This is the most probable explanation of the facts, and 

 is furthermore supported by the absence of lines of motion 

 or fracture, and as far as can be ascertained by a careful 

 microscopic investigation, also by the absence of those 

 minerals which are generally found along such lines in 

 rocks that have been squeezed. No clear proof of any 

 dynamic action is observable. 



We find, moreover, elsewhere, similar striped and banded 

 structures in certain basic intrusive masses which cer- 

 tainly have not been affected by pressure. The theralite 

 from Mount Royal, at the foot of which lies the city of 

 Montreal, is an example of this. This theralite breaks 

 here through the flat-lying silurian limestone of the 

 Trenton age, and probably forms the nucleus of an old 

 palaeozoic volcano. 



Although it cannot be maintained that the striped and 

 irregular banded structure which is so often found in 

 various basic rocks is never produced by dynamic action, 

 it may yet be shown that it often results from movements 

 in the mass before consolidation. It is probable that the 

 structure usually originates in this way ; but cases are 

 rare where the conclusion that the structure results from 

 dynamic action is quite excluded. It may be here also 

 remarked that no satisfactory reason can be assigned for 

 the sudden alterations in the size of grain which we so often 

 observe in gabbros and allied rocks. This can hardly 

 be accounted for by irregular cooling, as the tempera- 

 ture must have been practically the same in adjacent 

 parts of the magma. The cause may perhaps be looked 

 for in the great abundance of moisture in certain places. 

 If such be the case, however, we must reject the theory so 

 frequently held that the presence of "agents mineralisa- 

 teurs " exerts a slighter influence ' in the crystalline 

 development of basic magmas than on that of the acid 

 ones in which such an alteration in the size of the grain 

 does not usually occur to such an extent. 



