THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE 85 



a diameter of a foot or more, but according to local reports the pits 

 have not been worked during the last decade. As regards the age of 

 these dikes, their peripheral grouping round the margin of the 

 granite shows them to have originated from that source, and the 

 development of apatite suggests that they are of approximately the 

 same age as the more abundant tourmaline dikes, and hence like 

 these, later than the still more freciuent and simpler quartz-feld- 

 spar dikes. 



All the tourmaline and apatite dikes are undoubtedly to be re- 

 garded as indicating the final stage of igneous activity in this region, 

 and the introduction of the extreme products of differentiation of 

 the granite magma. The succession based on observed age rela- 

 tions is (i) granite gneiss, accompanied and followed closely by 

 (2) quartz-feldspar pegmatite dikes, which were succeeded after a 

 somewhat longer interval by (3) the rare element dikes containing 

 tourmaline or apatite. The sequence is such as indicates a natural 

 and progressive differentiation in an acid magma whose content of 

 mineralizers was low. Accordingly the two groups of pegmatite 

 dikes form a normal postintrusive suite, and though of distinct 

 and separate age, are genetically related to the same primary intru- 

 sive magma, namely, that of the post-Grenville pink granite gneiss. 

 If this is not the case, there must have been more than one such 

 intrusive with accompanying groups of satellite dikes. While this 

 can not be definitely disproved, the substantially uniform Hthologic 

 and structural conditions which obtain with respect to the granite 

 areas afford little basis for the assumption of a multiple invasion. 



If this be granted, that is, that there was only one period of 

 granite eruption, and that the pegmatites all belong essentially to 

 this period of invasion, then it follows that the emplacement of the 

 granite sills and bosses practically marks the end of the post-Gren- 

 ville igneous activity in this district. The gabbros and amphibolites 

 are post-Grenville ; the granites and pegmatites are likewise post- 

 Grenville; differentiates supposedly of the granite formation are 

 known, in two instances at least, to have cut the amphibolites. The 

 conclusion, therefore, on the basis of the arguments and data set 

 forth above, that the granite is postgabbroic in age, seems inevita- 

 ble. This view seems sanctioned also by other considerations, such 

 as the greater amount of metamorphism in the gabbro, which is 

 nearly everywhere altered to amphibolite, and by the universal 

 abundance of black inclusions in the granite, some of which may be 

 of gabbroic origin. 



