﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 3 



here as farther east, in fact there seems to be a slow but progres- 

 sive increase in severity of metamorphism in passing east. The 

 differences in this respect are not so prominent in the Grenville 

 and Laurentian rocks as in the later igneous rocks, but character- 

 ize all. Even here, however, the character of the metamorphism in- 

 dicates a considerable depth for the rocks concerned during the 

 time when it took place. But it also suggests a less depth of over- 

 lying material than is possessed by the region farther east. 



This overlying material has since been removed by slow surface 

 erosion. Greater thickness has been removed on the east than on 

 the west apparently, the differences in metamorphism being thus 

 most readily explained. Further, this removal by erosion took place 

 wholly in Precambric time indicating that the region was a land 

 area for a long period. Precambric time however was very long, 

 the Grenville sediments were deposited early in it, the district sub- 

 sequently rose above sea level and remained as land during the 

 long ages of the middle and late Precambric. The large amount 

 of rock thickness removed not only argues for a long erosion in- 

 terval but likely indicates renewal of uplift on one or more oc- 

 casions, since it is not probable that the region ever attained an 

 altitude as great as that represented by the thickness of rock re- 

 moved. 



Late in Precambric time, and toward the close of this long, 

 erosion period, came renewed igneous activity, an upward move- 

 ment of heavy, black, basic lava taking place. Not improbably 

 some of this material reached the land surface of the time and 

 spread out as lava flows. If so subsequent wear has removed 

 every trace of their presence, cutting away the surface sufficiently 

 so that the only sign of this igneous activity which remains on 

 the surface of today is the trap dikes, the lava-filled channels of 

 ascent of the molten rock. The trap is absolutely unmetamorphosed 

 and gives every indication of having solidified at quite shallow 

 depth. Hence the conclusion is forced that the eruption occurred 

 toward the close of the long Precambric erosion period previously 

 described, and since only a comparatively slight amount of wear 

 followed, that these dikes are of very late Precambric age ; in fact 

 it is by no means impossible that they may be as young as the 

 early Cambric. 



If we could follow these dikes down into the earth beneath the 

 surface of today, no doubt we should find that they lead upward 

 from underground masses of trap of considerable size, quite analo- 

 gous to the bathyliths of the earlier granites. 



