496 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



be no doubt of Ike unconforniability of these formations, approacliing each other 

 as they do with a peiHisterxt opposite dip and somewhat different strike. Un- 

 conformability has been shown to exist between the Laureiitian and Huronian 

 in Michigan, but this is the first time it has been proven in Wisconsin." 



Of the same supposed unconformability at Penokce Gap, Prof. K D. 

 Irving remarks (Am. Jour. Sci., 1877, (3) XIII., p. 308) : 



"The crystalline rocks of Wisconsin include unciuestionably two distinct 

 terraces, the one lying unconformably upon the other, as is beautifully shown 

 at Penokee Gap, on Bad river, in the Lake Superior country. Ilei-e a white 

 siliceous marble of the Huronian, overlaid by hundreds of feet of distinctly 

 bedded slaty rocks, and dipping northward, is to be seen within 'twenty feet 

 of large ledges of dark colored amphibolic gneiss, whose bedding planes dip 

 southward and strike in a direction diagonally across that of the more northern 

 beds. There are no doubt instances where the two scries are difficult to sepa- 

 rate, similar rocks occurring in both groups, but the existence of the two is 

 incontestable.*' 



In the third volume of the Geology of Wisconsin (pp. 94, 98, l06, 

 116, 117, 248-250) accounts of the unconformability of the LaTirentiar 

 and Huronian are given, but the kind of contact when seen was not 

 observed. But if the Laurcntian rocks are eruptive, then of course there 

 would be unconformability. The proof advanced was, that the folia- 

 tion of the granite and gneiss dipped at a diiferent angle from that of 

 the Huronian rocks. Here, as in the case of the Keewecnaw scries, the 

 Wisconsin geologists failed to take into account the conditions neces- 

 sary to prove their points ; while Professor Irving, without giving any 

 evidence of value, made out a beautiful fault —on paper — at the Peno- 

 kee Gap. So far as can be judged from the evidence presented by these 

 geologists, it. appears that they have in Wisconsin the same structure as 

 exists in the Azoic of Michigan, namely, a series of mixed sedimentary 



and eruptive rocks. 



From the following extracts it will be readily seen that there are no 

 other than lithological grounds for assignhig these rocks to the Huro- 

 nian and Laurcntian : .that they are two distinct formations they en- 

 tirely fail to prove. 



Major T. B. Brooke states (Geol. of Wise, 1880, IIL, p. 468) : 



" No rocks affording to me the slightest suggestion of a conglonicritic struc- 

 ture have been found in the Laurcntian system, its rocks being always highly 

 metamorphosed, and often so much so as to destroy all traces or suggestions of 

 bedding." 



In the same report (p. 531) Mr. Brooks also points out several areas 

 of granite in which this rock was found to be intruded into the supposed 



