70 JOUmAL OF GEOLOGY— SUPPLEMENT 



pass the granite. Presumably this has been accomplished in many 

 cases and somewhat more basic rocks, granodiorite or diorite, have 

 been encountered. There is an abundance of diorite in the "grani- 

 tic" areas of the pre-Cambrian and possibly careful structural and 

 petrographic work would reveal that this diorite underlay granite. 

 This has already been shown to be true in the case of the Palaeozoic 

 Saugus bathoHth of Massachusetts.^ Seldom has sufficiently 

 detailed work beeh accomplished to reveal such facts in those areas 

 of the pre-Cambrian where igneous rocks dominate. In the case 

 of very large bodies erosion might penetrate two or more miles 

 and then reach only types not very different from granite, say 

 granodiorite or quartz diorite, which are not distinguished from 

 granite in a good deal of areal pre-Cambrian work. Probably only 

 profound erosion would reach the more basic t3^es and it may 

 reasonably be doubted whether erosion is ever sufficient unless 

 the igneous mass is favorably disposed, as it was in the case of the 

 Duluth laccolith. It is true that great depth of erosion is some- 

 times believed to obtain in pre-Cambrian areas, but this appears 

 to be often merely the attributing of great power to geologic forces 

 in the distant past. On the contrary, when the structural evidence 

 is examined there has often been found but moderate erosion. 

 Thus, in the Thousand Islands region Cushing has shown that some 

 batholiths have been barely de-roofed,^ and in the Bancroft area 

 of Ontario there are still remnants of effusive rocks, comagmatic 

 with the batholiths, which were formed on the surface about the 

 time of the intrusion of the batholiths. Clearly the surface at 

 that time could not have stood at a level greatly different from that 

 of the present surface. These are isolated cases, of course, but 

 infolded patches of surface volcanic rocks in the batholithic pre- 

 Cambrian areas are common enough to indicate that possibly these 

 terranes have not suffered excessive erosion. Apparently, then, 

 one would not expect erosion to have exposed areas of basic rocks of 

 batholithic dimensions even if the granitic batholiths were formed 

 by differentiation of basaltic magma. It is tentatively suggested, 

 however, that it may be the greater erosion of pre-Cambrian ter- 



' C. H. Clapp, op. cit., p. 6. 



2 "Geology of the Thousand Islands Region," N.Y. State Museum, Bull. 145, p. 43. 



