THE QUARTZ SLATE MEMBER. 179 



the foot-wall ot the mines of the GoifelMc district; while the loAvei' portions 

 of the forniMtion have often l)een exposed in test-pitting operations, so that 

 the course and width of the belt can be quite accurately laid down. It may 

 be noted that there are large exposures on the west side of Sec. 14, T. 47 

 N., K. 4(i W., Michigan, where the several railroad spuis running to the 

 Palms and Anvil mines have cut deeply into the slate. On the Black river, 

 in the northern jjart of Sec. IS, T. 47 N., K. 47 W., Michigan, are natural 

 exposures of the formation. To the east of Sunday lake, in Sec. 10, T. 47 

 N., R. 45 W., Michigan, where the Quartz-slate formation has an unusual 

 thickness and its outcrop belt an unusual width, there are again very large 

 and continuous exposures, displaying the entire thickness of the formation, 

 which liere reaches s(,)metliing like 80U feet. The Quartz-slate here makes 

 \i\) the bulk of two bold bluffs, one in the southwestern portion and tlie 

 other in the southeastern portion of Sec. 10, and extending thence into 

 Sees. 11 and 14. xVbout all the phases that are characteristic of the Quartz- 

 slate formation, with the exception of the chloritic and biotitic slates, are 

 displayed in these exposures. The last exposures of this formation met 

 with before reaching the Presque Isle river are those in the southern })art 

 of Sec. 17, T. 47 N., R. 44 W., Michigan. 



Mode of ikpositloii and mnrce of )iKd<')iaL — It has already been made e\d- 

 dent that the various rock phases of which the Quartz-slate member is com- 

 posed are of a detrital nature. Even the tinest grained phases and the finer 

 interstitial material of the coarser phases, although at times not now plainly 

 showing their fragmental character, are taken, notwithstanding this, to be 

 wholly of detrital origin, being com})osed of the same materials as the 

 coarser phases, either in the original unaltered condition, or, as is often the 

 case, somewhat changed by metasomatosis, the chlorite, kaolinite, micas, 

 and finely crystalline (piartz being in the main secondary derivatives from 

 feldspathic detrital material. 



The nature of the most of the detritus of which this formation is mainly 

 compo,sed is such as to suggest very strongly the derivation of its material 

 from some sort of granitic or gneissic rock. The pieces of (juartz, orthoclase, 

 niiciocline, plagioclase (probably ali)ite and oligocla.se), and mica, are in 

 their association and in their peculiarities just what we would expect in a 



