F. H. Lahee — Metamorphism and Geological Structure. 467 



intrusion and of deformation were in part contemporaneous, 

 the forces active in the folding may have assisted in determin- 

 ing the shapes and directions of the dikes ;* and, (7) that, not 

 being the cause of the regional metamorphism of the Basin,f 

 and, moreover, having its inception during deformation, the 

 process of intrusion may have been a consequence of the action 

 of the deforming forces. 



Relations between the minette dikes and the acid in- 

 trusives. — Because the minette dikes and the Boston Neck 

 granite entered the Carboniferous strata during the period of 

 deformation, there is strong reason to suppose that they may 

 be related to one another. Someone has suggested that the 

 pegmatites and quartz veins, on the one hand, and the 

 minettes, on the other hand, represent complementary poles 

 of differentiation. Advocates of this view would have to 

 explain (1) why the minettes are so often intersected by quartz 

 veins w T hich are almost certainly of the Acid Intrusive Series ; 

 (2) why there are so few minette dikes as compared with the 

 vast number of pegmatite dikes and quartz veins ; and (3) why 



sions of it. The granite was erupted (sic) after or during the folding of the 

 schist, otherwise it would have become a gneiss" (p. 51). We find in 

 Geikie's 'Text-Book' that the granites of southeastern Ireland were injected 

 into Lower Silurian rocks after the latter were folded (p. 726). Finally, 

 we may quote from T. A. Jagger and E. Howe (The Laccoliths of the Black 

 Hills, U. S. G. S., Ann. Rept. 21, Pt, III, 1901): "Throughout the Rocky 

 Mountains igneous phenomena have accompanied colossal movements of 

 uplift, folding, and faulting" (p. 187). "The history of intrusion in the 

 Black Hills is believed to be intimately associated with the history of the 

 larger deformation. Intrusion is not conceived to have been in any sense a 

 cause of the greater uplift, but an effect" (p. 282). "Cross recognized in 

 the Mosquito Range and Tenmile district the influence of orographic move- 

 ments concomitant with intrusion" (p. 286). "In many places (in the 

 Rocky Mountains) intrusions accompanied or followed the greater move- 

 ments. . . " (p. 287). 



* Compare A. Harker :....' 'When very large bodies of magma are 

 intruded under a considerable cover, their form and disposition may be 

 determined mainly by the distribution of stress which thus finds relief, with 

 very little regard to the structure of the encasing rocks" (Natural History 

 of Igneous Rocks, N. Y., 1909, p. 83). 



\ G. F. Loughlin wrote : "The details of metamorphism in the Kingstown 

 area have not been exhaustively studied, but it is very evident, from the 

 field work done, that vertical dips and the most complete recrystallization of 

 the sediments are found where granitic intrusions are most abundant. 

 There seems, then, no reason for doubting that in the Kingstown area .... 

 the granite intrusion accompanied metamorphism and folding. As the 

 Kingstown sediments have been determined to be of Carboniferous age, the 

 time of granite intrusion and folding may be correlated with the Appala- 

 chian Revolution." (Intrusive Granites and Associated Metamorphic Sedi- 

 ments in Southwestern R. I., this Journal, xxix, 447, 1910, p. 445.) The 

 first statement seems to lead, one to infer that the metamorphism of the 

 sediments was largely due to intrusion ; but, as we have attempted to ex- 

 plain, we regard the amount of metamorphism primarily caused by the 

 intrusion to be slight in comparison with that due to folding and concomitant 

 shearing, together with the static after-effects. 



