THE UPPEK SLATE MEMBEll. 313 



large grains of feldspar. It would seem from this section tliat tlie line grained mica- 

 schists in whicli the fehlspar is decomposed to a much greater extent tliaii in this 

 rock, and wliich contain no large fragments of quartz, must originally liave been feld- 

 spatiiic sediments. 



9. Black biotitic slate, from east side of fault and near base of formation. Speci- 

 men 95(59 (slide 337S), from 1700 N., 1000 W., Sec. 14, T. 44 N., 1{. li W., Wisconsin. 



A black fine grained compact rock, wliich breaks with a conchoidal fracture. 



Very small clastic particles of fpnirtz and fcldsi)ar compose one-half of the area 

 of the section. The quartz grains are ordinarily distinctly enlarged, while quite a 

 good many of the grains of feldspar are tolerably fresh. The greater number of 

 them are altered to some extent to biotite, while frequently they are almost conq)letely 

 thus altered. All stages of this process are seen, and while the particles of felds]iar 

 and secondary folia of biotite are very small, the transformation is distinctly made 

 out. Till- inter.stitial material composing the other half of flic section consists of 

 exceedingly fine crystallized quartz, of minute folia of biotite, and of an abundant 

 black opaque material which is doubtless ferriferous and possibly also carbonaceous. 

 All of the biotite in the section is believed to be of secondary origin. 



10. Bliick biotitic slates, from we.st side of fault and at the lower middle horizon. 

 Si>eeimens 9.>.")0 (slide 3322) and 1480 Wis., (slide 207), from .JOO N., 1400 W., Sec .11, 

 T. 44 N., 11. 3 W., Wisconsin. 



Fine grained and finely laminar rocks, which cleave readily along the planes of 

 lamination. Oontained in the fine material are numerous roundish black lustrous 

 cleavage areas, which are taken to be large fragmental particles of feldspar. A lens 

 shows very numerous minute crystals of pyrite or pyrrhotite, probably the latter, 

 for the rock gives, finely pulverized, a magnetitic powder, which may, however, be 

 due to magnetite. 



The thin sections show the rocks to consist of two parts, a finely crystalline 

 matrix and coarse well rounded fragmental feldspars. These felds])ars have always 

 altered to a greater or less degree, the alterations resulting in the formation of tiie 

 biotite, many .small folia being always found in a single individual of feldspar. 

 Every gradation of the change is seen from grains of feldspar which contain but 

 few folia of biotitic to those in which the remaining feldspar is just siitlicient in quan- 

 tity to enable one to perceive that the detached areas are parts of one individual. 

 Doubtless also the alteration to biotite and (piartz has completely taken place in 

 many cases, when the outlines of the (U-iginal clastic areas would be entirely lo.st. 

 Accompanying the biotite secondary to tlu; feldspar is a large anTount of black 

 opaque material in minute particles. This is in all probability also secondary, as its 

 amount iiu^reases as tlie (piantity of biotite increases. The black roundish spots 

 sjiokeii of under the macroscoi>ic descriptions are evidently these altered fragmental 

 feldspars. The matrix of the roek is eoiiqiosed of intimately mingled quartz, feld- 



