76 THE PENOKEE lEON-BEAKING SEKIES. 



Las been shown to exist between the Lanrentian and ITnroniau in Michif,'an, but tliis 

 is tlie first time it has been jn'oved in Wisconsin." 



Of the same supposed uuconformability at Penokee gap Prof. 11. D. Irving 

 remarks (Am. Jour. Sci., 3rd ser., vol. xiii, 1877, p. 308) : 



"The crystalline rocks of Wiseousiu inclndi; unquestionably two distinct ter- 

 ranes, the one lying unconforniably upon the other, as is beautifully shown at 

 Penokee gap, on liad river, in the lake Superior country. Here a white siliceous 

 marble of the Uuronian, overlaid l)y liundreds of feet of distinctly bedded slaty 

 rocks and dipping northward, is to be seen within 20 feet of large ledges ol' dark 

 colored amphibolic gneiss, whose bedding planes dip southward and strike in a direc- 

 tion diagonally across that of the more northern beds. There are no doubt instances 

 where the two series arc dilhcult to separate, similar rocks occurring in both groujjs, 

 but the existence of the two is incontestable." 



In the third volume of the Geology of Wisconsin (pp. 94, 08, 108, IIG, 117, 

 248-250) accounts of the unconformability of the Laurentian and Uuronian are given, 

 but the kind of contact when seen was not observed. But if the Laurentian rocks 

 are eruptive, then of course there would be un<!onformability. The ])roof advanced 

 was that the foliation of the granite and gneiss dijiped at a different angle from that 

 of the Huronian rocks. Uere, as in the case of the Keweenaw series, the Wisconsin 

 geologists failed to take into account the conditions necessary to i)rove their i)oints, 

 while Prof. Irving, without giving any evidence of value, made out a beautiful fault- 

 on paper — at the Penokee gap. !So far as can be judged from the evidence i)resented 

 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 seeu that there are no other than 

 lithological grounds for assigning these rocks to the Uuronian and Laurentian; that 

 they are two distinct fornmtions they entirely hiil to prove. . . . 



In 1880 Professor Irving gives as the reasons for assigning the rocks which are 

 placed in the Laurentian in Wisconsin to that system, their " close lithological sim- 

 ilarity — the only nuirked dilfcrence being the absence of crystalline limestones in the 

 Wisconsin area — of similar structural relations to the Uuronian, Keweenawan, and 

 Lower Silurian systems, and of probable direct continuity with the Canada Lauren- 

 tian through the u])per peninsula of .Michigan and underneath the waters of lake 

 Superior." (Trans. Am. Inst. Miu. Eng., vol. Viii, 1880, pp. 480, 481.) 



Of the Uuronian in the same article it is st.itetl (p. 483): 



"The rocks of this scries have been called Uuronian by Brooks, and, in the 

 writer'sjinlgment, correctly so, on account of their similarity to the Canada Uuronian, 

 with which they not improbal)ly lia\'c a direct connection nudcrncatli the Silurian of 



