R. D. Irving — Is there a Huronian Groiip f 251 



rocks by the granite. It is true of course that these granitic 

 intrusions might be in part quite independent of, and of later 

 date than the great granitic masses ; but, as Dr. Kominger 

 argues, and as our own observations have abundantly shown, 

 these intrusions of the schists along their contacts with the 

 granite are so plentiful in certain parts of the region that it 

 seems necessary to conclude that such invaded schists are older 

 than the great granitic masses themselves. On the other hand, 

 we have found certain of the contacts of the stratiform rocks 

 with the granite to present admirable examples of basal con- 

 glomerates, the granitic and gneissic rocks having evidently 

 been beaten upon and broken down by the sea in which the 

 other rocks were laid down. 



These entirely contradictory appearances seem, however, to 

 find a very simple explanation in the view that the stratiform 

 rocks themselves are, in fact, made up of two entirely distinct 

 sets ;* an older series of very intensely altered and crumpled true 

 crystalline schists, in the main of a greenish color, which are in- 

 vaded intricately by the granite at their contacts with it ; and 

 a newer, feebly altered, mainly fragmental series, whose con- 

 tacts with the granite and the schists of the older basement are 

 such as to render the intervening structural break very evident. 

 The contrast between these two sets of stratiform rocks as to 

 degree of alteration, and as to the penetration of the one set by 

 the granite while the others show no such penetration, was 

 partly realized by Dr. Kominger, and is plainly stated in his 

 report on the northern peninsula of Michigan, in which, 

 however, he yet regards the granite masses as newer than 

 all the stratiform rocks of the region, the high degree of 

 alteration of the schists penetrated by them, as compared with 

 the smaller degrees of alteration of the higher strata, being ex- 

 plained by the greater proximity of the one set to the granite. 

 In the same report Dr. Kominger shows, however, that the 

 relatively unaltered strata often come directly in contact with 

 the granite, in which case they present a peculiar appearance 

 suggesting to him a slighter alteration by contact with the erup- 

 tive masses. The peculiar granitoid quartzites which he takes to 

 have been produced by the metamorphosing action of the 

 granite are plainly, however, merely detrital derivatives from 

 the granite, and often run into coarse bowlder-conglomerates, 

 as Dr. Kominger himself now realizes and states.f However, 

 neither he nor any other writer on this region seems to have 

 realized heretofore that the clue to its geology lies in the 

 separation of the stratiform rocks into two entirely distinct and 

 mutually discordant series. 



* As first suggested by myself in 1883, (Fifth Annual Report TJ. S. G. S., p. 

 190) and more definitely announced in 1884. 



f Unpublished MS. report on the Geological Survey of Michigan, 1884. 



