216 R. D. Irving — Is there a Huronian Group f 



Returning now to the classification of rocks and the charac- 

 terization of the Group, quoted at the beginning of this paper, it 

 will be seen that under this classification the Huronian can only 

 be a group, since — (1) it is essentially non-crystalline, and there- 

 fore not to be placed with the non-eruptive crystallines, from 

 which, moreover, it is structurally wholly separate; (2) it is truly 

 clastic and sedimentary, and therefore must be included with the 

 clastic groups, because these " comprise all the formations of 

 known clastic origin ;" and (3) it has an immense volume — a 

 volume not only comparable with those of the ordinarily recog- 

 nized fossiliferous groups, but in excess of the volumes of 

 many of them. It cannot be a formation, being made up of 

 formations. The classification then leaves us no choice but to 

 call it a group. 



The definition quoted demands, it is true, some other charac- 

 ters for the group in addition to those just mentioned, but 

 since the latter are restricted by it to the true group, it is evi- 

 dent that in the original Huronian we are dealing with one of 

 these groups, or that the definition is defective. This point I 

 return to in the closing section of my paper, and may therefore 

 pass it over for the present, merely remarking by the way that 

 the question which has been raised by some, as to whether the 

 Lake Huron Huronian may not be the equivalent of some 

 group elsewhere recognized as fossiliferous, say the Cam- 

 brian or the Silurian, is hardly worthy of discussion. Here, in 

 the typical region itself, the Upper Cambrian sandstone 

 (Potsdam) may be seen crossing the immensely denuded sur- 

 face and decapitated folds of the Huronian ; so that to include 

 the Huronian even in the Cambrian would require us to stretch 

 that term over one of the greatest, if not the greatest, of the 

 structural gaps in the whole geological column. Moreover, 

 there is every reason to believe that the great Keweenaw Series 

 of Keweenaw Point belongs to this interval, though by no 

 means filling it. This series, on the south shore of Lake Supe- 

 rior, lies unconformably beneath the same Cambrian sandstone 

 which traverses the edges of the Lake Huron Huronian, while 

 at the same time unconformable to rocks beneath it whose 

 equivalence with the original Huronian there is every reason 

 to accept, as is subsequently maintained in this paper. 



[To be continued.] 



