GEOLOGICAL EXPLORATIONS AND LITERATURE, 53 



in the form of a diary, Avith the results of a microscopical study of thin sec- 

 tions in^ers[)ersed. These details have often been used in the compilation 

 of our maps herewith, most of the important outcrops described by Mr. 

 Wright having-, however, been rev-isited for our work. Wright's route of 

 travel also is laid down on our maps. 



The second of Mr. Wright's chapters is of a more particular interest, 

 because in it he gives his views as to the general stratigraphy of the Iron- 

 bearing series, and as to its relations to the adjoining formations. The fol- 

 lowing quotations from this chapter will serve to show Mr. Wright's con- 

 clusions ; the omitted portions are mainly microscopical details: 



In this geological section it has been my endeavor to correlate the Penokee Iron 

 bearing series — Lower Ilnroiiian — as near as possible with those of Michigan, assumed 

 to be of the same age, as they undoubtedly are. 



We will adopt, for convenience of reference, the numbering first emploj'ed in the 

 Geological Report of Michigan, 187.3, to designate the Lower Huronian beds. 



The first rocks we will consider, however, are those of the Laurentian series, 

 found outcropping on the south side of the Penokee range. These rocks are granites, 

 gneissoid gi'auites, and gneisses. At the Gap, they have a strike a little to the south 

 of west, and dip from G5oto 80° to the south. The granites are dark gray to reddish, 

 depending on the predominance of the mica or feldspar. They are fine to very 

 coarse grained, the medium grained varieties being, however, the most common. The 

 essential mineral ingredients are usually easily recognized. The bedding planes, or 

 "grain" of the granite, even in the massive varieties, may, generally, after careful 

 examination, be made out . . . 



Orthoclase is the prevailing member of the feldspar family. . . . 



The mica is chiefly biotite and of a dark color. . . . 



The gneisses, like the granite, vary fi'om dark gray to reddish. Usually they are 

 distinctly laminated with the layers of quartz, feldspar, and folia of mica. It is some- 

 times slaty, but more generally it is heavily bedded, and passes almost imperceptibly 

 into a gneissoid or a massive granite. The dark, finegrained, slaty varieties resemble 

 hornblende-gneiss and hornblende schist; in tact, I have been obliged to make micro- 

 scopic sections before being able to decide which it was. . . . 



Nonconformably overlying the Laurentian rocks are those of the Huronian 

 series. We have at the Penokee Gap, where the Wisconsin Central railroad ciosses 

 the range, one of the best opportunities I have ever chanced upon for observing this 

 interesting fact. This nonconformability on the Penokee, I believe, was first noticed 

 by myself in 1875. The Huronian series at the Gap are plainly bedded, and have a 

 strike of a little north of east, and dip very uniformly (J6° to the north. 



