THE UPl'EK SLATE MEMBER. 309 



this head. At TyU-rs fork and vieinit}- then^ m\' quite a niiml)or of expo- 

 sures of cldoritic and Ijiotitic shites and but a single one of pure chloritic 

 shxtes. West of Tylers fork the schists and slates are almost wholly mica- 

 ceous, although chlorite is yet occasionally found. There are, then, precisely 

 the same stratigraphical relations with reference to chlorite and mica in the 

 slates and schists that there are in the graywackes and graywacke-slates. 

 Tlie two classes of rocks ai'e also interlaminated with each other. In Sec. 

 1 4, T. 46 N., R. 2 H, a chlorite-schist is but a short distance from an expo- 

 sure of typical gray wa eke. At Potato river again the one biotite-slate 

 there found is interstra titled with graywackes and graywacke-slates. At 

 Tylers fork the gra^'^ackes and graywacke-slates and the mica-schists and 

 mica-slates are interstratified in the most intimate manner, both rocks occur- 

 ring at times in tlie same exposure. Beginning at the top of the Upper 

 slate member, the order of succession of the different jjhases of rock as here 

 found, as taken from the tabulations, are biotite-slate, chloritic graywacke, 

 chloritic biotite-slate, biotitic graywacke, l)iotitic graywacke-sLate, chloritic 

 graywacke-slate, chloritic graywacke. At Bad river, the order, beginning 

 at the top, is as follows: mica-schists, micaceous graywacke, biotite-schists, 

 micaceous graywacke, mica-slates. West of Bad river, in the vicijiity of 

 English lake, mica-schists and mica-slates only are found. 



TABULATION OP PETROGRAPHICAL OBSERVATIONS. 

 Exposure in Sec. 18, T. 44 N., R. 3 W., Wisconshi. 



1. Biotite-slate. Specimeii 167 Wr., from 1,000 N., 1,-I50 W., Sec. IS, T. 44 N., 

 R. .3 W., Wisconsin.' 



Tlie rock contains, in an apbanitic background, very uuuierous small crystal sur- 

 faces which feebly reflect the light, is black, and cleaves somewhat irregularly parallel 

 to the bedding. 



The thin section is composed of a finely crystalline backgnmnd and individuals 

 and clusters of individuals of feldspar, in about equal proportion. The background 

 consists of finely crystalline quartz, snmll brown folia of biotite, very numenms black 

 particles of ferrite or biotite, or both, and probably also some carltonaceous material. 



' The numbers of specimens and .slide.s are usually those of the coUcetion of the lake Superior 

 division. SiX'ciuieii.s with Wr. after tlu' numbers are from, the collection of the hit(^ Mr. Charles E. 

 Wright. S])ocimens with Wis. al'ter the numbers are from the eoUeetion of the Wisconsin Geoh)gical 

 Snrvey. Locations are fiivin fioni the s(>utlica«t corner of the sectiim in steps of 2,(tO() per mile. 



