252 It. D. Irving — Is there a Huronian Group ? 



The proofs of this distinctness are analogous to those which 

 establish the separateness of the type Huronian, of Lake Huron, 

 from the adjoining crystalline schists, viz : (1) a general litho- 

 logical contrast between the two series concerned, the one being 

 mainly but little altered, the other highly crystalline and schis- 

 tose ; (2) visible discordances in a few places on the contacts of 

 the two sets of rocks ; (3) the penetration of the lower strata by 

 granite veins and masses which fail to penetrate the higher 

 detrital rocks, but on the contrary have yielded fragments to 

 them ; (4) the development of true basal conglomerates at the 

 contacts of the two series ; and (5) the fact that the higher 

 detrital rocks are in contact with different members of the lower 

 series. 



Nevertheless there are presented here some differences from 

 the conditions observable on the north shore of Lake Huron, 

 which serve to obscure the true relations of the different rocks. 

 The placing together of all of the stratiform rocks of the Mar- 

 quette belt by most geologists is not at all to be wondered at, 

 for here the newer detrital rocks have been subjected to a much 

 greater lateral pressure, and are consequently much more highly 

 inclined and closely folded ; besides which this closer crumpling 

 has at times developed a tendency to a transverse cleavage ac- 

 companied now and then by the development of some of the 

 micaceous minerals which are so frequent accompaniments of 

 pressure metamorphism. Thus the evidence under the first of 

 the heads just named is in part obscured, more especially 

 through the development from fragmentals of certain mica- 

 schists, though the essential lithological differences between the 

 two sets of rocks become distinct enough on closer inspection. 



A more important obscuration of original conditions applies 

 to the evidence under the second head. Discordances are very 

 plainly to be seen in a few places, - * and perhaps in general the 

 higher series lacks the high inclinations of the lower ; still there 

 are man}^ places where its inclinations are very high, and where 

 the common pressure, to which both underlying schists and 

 overlying detritals have been subjected, has made them nearly 

 or quite parallel. This apparent obliteration of unconformity 

 is yet more marked in some of the districts subsequently 

 to be noted. To render the case still more difficult of com- 

 prehension, the denudation which has so deeply truncated 

 the region has occasionally brought to light within the area 

 occupied by the newer detrital strata patches of the older schis- 

 tose basement; which patches, sometimes completely sur- 

 rounded, or bordered closely on either side by the newer frag- 

 mentals, it has seemed necessary to regard as parts of the same 



* As for instance on the N."W. J of sec. 1, township 47, range 24 west, south of 

 Marquette. 



