THE UPPER SLATE MEMBEE. 311 



These sections arc lik«^ -S, exeejit that tlioy arc somewhat eoarser grained and 

 contain more innscovite and approach nearer (especially slide 27) to a typical mica- 

 schist. 



5. Biotite-schist, tiom an npper liorizcm. Specimen 153 Wr., 335 N., 1,050 VV., 

 Sec. 4, T. 44 N., K. 3 W., Wisconsin. 



The rock is daxk gray, of a ratlier fine, nniform grain, finely laminated, yet so 

 compact as to break quite readily across the jilane of lamination. The rathei- large 

 black abundant flakes of mica give the specimen the appearance of a t.\^)ical mi»;a- 

 schist. 



The thin section shows this rock to be a mica-schist. It has an interlocking 

 quartzose background, mingled with which in subordinate (juantity are both ortlnj 

 clase and plagioclasc. Contained in this background is much biotite in tolerably 

 wide, long, well defined blades, which cut through both quartz and feldspar. No 

 grains of quartz are found whicli are plainly enlarged. This rock is the most com- 

 pletely crystalline of any mica-schist in the formation. It contains the merest trace 

 of old feldspar grains, no quartz which can be showoi to be fragmeutal, and a consid- 

 erable quantity of fresh feldsjtar. In short, if this schist has formed, as from its posi- 

 tion we are obliged to believe, from a fragmeutal rock, no trat^e of the alteration is 

 discoverable. The fresh feldspars in all probability, in this, as in other cases where 

 they are jiresent, are of secondary origin, not original fragmeutal particles. (PL 

 XXXIV, Fig. li.) 



0. Biotite-schist, from an uppermost horizon. Specimen 154 Wr., 500 N., 1,000 

 W., Sec. 4, T. 44 N., 11. 3 W., Wisconsin. 



This rock differs only from 153 Wr. in that it is finer grained. 



In section the chief constituents are quartz, biotite, and feldspar, the first being 

 preponderant, and the last .comjirising both oithoclase aiid plagioclase. The section 

 contains al.so quite a quantity of black opaque material. .Many of the grains of 

 quartz have undergone secondary enlargement. That all tlic l)lotite is secondary is 

 probable, although only a portion of it can be shown to be of tliis nature. 



From the section at Bad river and vicinity. 



7. Garnetiferous biotite-slates, from west side of fault at base of formatiou. 

 Specimen !».~.52 (slide 31S7), N., 1,800 W\; Specimen 9.'i.')4 (.slide 3189), N., 1,7.W 

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



These rocks are of a reddish or grcenisli black color, aplianitic, finely laminated, 

 and readily cleave along the plane of lamination. The cleavage surfaces are lustrous 

 aiul covered by many small protuberances, which are taken to be due to contained 

 garnets. » 



The sections arc mostly composed of small Hakes of biotite with particles of a 

 black substance set in a tine quartzose groundmass. The (|uartz is iu fine and closely 



