THE UPPER SLATE MEMBER. 303 



In the subsequent (letailed descriptions of the rocks of this member 

 they are grouped into several areas geographically. The distriVjution cor- 

 responds in a loose way to the classification of the rocks above given. At 

 the west end of the member the mica-schists and mica-slates are the only 

 rocks found. At Bad river and vicinity these rocks greatly predominate. 

 In passing eastward micaceous graywackes and graywacke-slates, chloritic 

 graywackes and graywacke-slates, and clay-slates are eacli in turn tlie pre- 

 ponderating rocks. In tlie transition from one type of i-ock to tlie next 

 one there are cross sections in which two or more of tliese kinds may 

 be found. 



Before passing to the petrographical characterization of the types of 

 rocks of this group, it will perhaps be well to give the kinds of rocks found 

 in the geographical divisions as given in the tabulations. All the rocks of 

 the English lake, Penokee gap, and Mellan junction sections, and also the 

 exposure in Sec. 18, T. 44 N., R. 3 W., Wisconsin, have as their chief con- 

 stituents quartz, feldspar, and mica. -. The rocks in Sec. 1 8 and those in the 

 English lake section are all mica-schists and mica-slates. At Penokee. gap 

 are some graywackes and tliin belts of (juartzite, Imt both are micaceous. 

 At the Tylers fork section the mass of the rocks are graywackes and 

 graywacke-slates. However, in exposure all show lamination with suffi- 

 cient plainness to enable one to readily determine strike and dip. The 

 difference, then, between graywackes and graywacke-slates is of minor 

 importance, and is used only for convenience to show relative degrees of 

 massiveness. The rocks at the section between Tylers fork and Potato 

 river and at the section in the vicinity of Potato river differ in no respect 

 from those at the Tylers fork section, with the exception that biotite is less 

 and chlorite more plentiful. Also, at one exposure in the extreme northern 

 part of the meml)er are found some conglomerates and (juartzites, whit-]i 

 will hv further refencd to. The rocks in the section located in tlie nortli 

 part of T. ■^F^ N., R. 1 E., and ;dong the west range line of R. '2 E., 

 Wisconsin, are mostly chloritic graywackes, although tlien^ are also 

 found magnetitic clay-slates. The rocks, tlien, differ from tliose found 

 at Tylers fork and Potato river in that l)iotite has almost entirely 



