108 THE PENOKEE lEON-BEARING SERIES. , 



are not in conformity with tliat series.' The chief constituents of tlie 

 gneisses are feklspar, quartz, hornblende, biotite and quite frequently 

 magnetite, epidote being an accessory. In certain of the specimens l)io- 

 tite is absent and in others hornblende. ' The relations of the minerals are 

 those ordinarily found in tj^pical crystalline schists, which give little or no 

 indication of their original condition. Some of the gneisses, however, show 

 that the biotite has in a large measure been derived from feldspar. The 

 alteration of each grain of the latter mineral to many folia of mica, with 

 the simultaneous separation of quartz, presents exactly the appearance sub- 

 sequently described (chapter vi) as characteristic of the fragmental feld- 

 spar I'ocks of the Upper slate belt, which there earned to the extreme has 

 resulted in forming from a fragmental rock a completely crystalline mica- 

 schist. There is this difference, however: In the Upper slate the crys- 

 talline schist was traced back into an unmistakable fragmeiital i-ock, \\hile 

 here the origin of the rock is unknown. In these gneisses nuu-li of the 

 cpiartz and feldspar has a general roundish appearance, which suggests a 

 fragmental origin. The hornblende-gneisses, upon the other hand, associ- 

 ated with these biotite-gneisses contain conq)aratively little or no quartz, a 

 good deal of hornblende, and plentiful magnetite. The last are, then, rocks 

 which are presumably schistose eruptives.' 



Between Bad river and Potato river the country south of the Penokee 

 series is low, swampy, and almost without exposure, so that little is known 

 of the character of the rocks. One exposure of an obscure chlorite-gneiss 

 is found in Sec. 5, T. 44 N., R. 1 W., Wisconsin, the only noticeable tiling 

 about which is the occuiTence of a considerable quantity of tourmaline, a 

 rather uncommon accessory in the crystalline schists of this district. 



At Potato river, in contact with and just south of the Penokee rocks, 

 lar'ge exposures of the underlying schistose rocks are found. They are 

 here fine grained; some of them are nearly massive and others highly 

 schistose. They are seen to contain, microscopically, many small grains or 

 crystals of feldspar, roundish areas of chlorite, and sometimes large areas 



' For detailed descriptions of the exposures here see Geol. Wis., 1880, vol. in, pp. 93-9t!, and 

 p. 224. 



«A. Geikle: Recent work of the Geological Sjirvey in the Northwest Highlands of Scotland, 

 (juart. .Jour. Geol. Soc. London, vol, XLiv, 1SX8, pji. 387-399. 



