GEOLOGICAL EXPLORATIONS AND LITERATURE. 41 



It has been my constant aim, and still is, to correlate the Penokee series of rocks 

 with those of Michigan, and there exists in my own mind no reasonable doubt that 

 the rock formations of these two districts are the equivalents of each other. In the 

 Penokee we have the limestone and quartzite members; the belts of magnetic* schist 

 interlaminated with the greenstones; also the black slates and mica-schists, all 

 occupying relatively the same stratigraphical position as in the Michigan series. 

 (Pp. 22-23.) 



Irving (B. D.). Note on the Age of the Crystalline Rocks of Wisconsin. Am. 

 Jour. Sci., 3d series, vol. xiii, 1877, pp. 307-309. 



The object of this note is to oppose Bradley's view, then recently 

 expressed, and indicated also on his geological map of the United States, 

 that the crystalline rocks of Wisconsin and Michigan may be altered Lower 

 Silurian. It is a general outline statement of the succession of pre- Potsdam 

 formations in northern Wisconsin, as the following quotation will indicate: 



The crystalline rocks of Wisconsin include unquestionably two distinct terranes, 

 the one Ijnug unconformably upon the other, as is beautifully shown at Penokee gap, 

 on Bad river, in the lake Superior country. Here a white siliceous marble of the 

 Huronian, overlain by hundreds of feet of distinctly bedded slaty rocks, and dipping 

 northward, is to be seen within 20 feet of large ledges of dark colored amphibcjlic 

 gneiss, whose bedding planes dip southward and strike in a direction diagonally 

 across that of the more northern beds. There are no doubt instanc-es where the two 

 series are diflticult to sci)arate, similar rocks occurring in both groups, but the exist- 

 ence of the two is incontestable, and their unconforniability with the unaltered 

 Potsdam equally so. The facts proved thus far with regard to the older rock series 

 of Wisconsin may be briefly summarized as follows: The oldest (I) are gneisses and 

 granites with other rocks ; these are overlaid unconformably by (II) a series of quartz- 

 ites, schists, diorites, etc., with some gneiss and granite; these in turn are overlaid — 

 probably also unconformably, but this is not certainly proven — by (III) the Copper 

 series, which includes greenstones and melaphyres, and also great thicknesses of inter- 

 stratified sandstone, melaphyres, amygdaloids, and shales, the whole having a thick- 

 ness of several miles; these finally are unconformably covered by (IV) a series of 

 unaltered horizontal sandstones including numerous fossils, many of which are closely 

 allied to those of the Potsdam sandstone of New York, and all of which have a marked 

 Primordial aspect. I and II are referred to the Laurentian and Huronian systems of 

 Canada, because they bear the same relations to one another and to the Copper series 

 that these systems do. (P. 308.) 



1S78. 



Irving (R. D.). Report to T. C. Chamberlin, State Geologist, of work done in 

 the Penokee region in 1877, dated December 21, 1S77. lu Annual L'eport of the 



