180 THE PENOKEE IROlfBEAEmCr SERIES. 



granitic detritus. And more than this, they are entirely similar to the par- 

 ticles of these same minerals as they appear in granite and gneiss which is 

 so largely exposed immediately to the south. One of the most noteworthy 

 things about the formation (excepting the upper horizon) i.s' the relatively 

 small degree of assortment that the detrital material of which it is composed 

 has received. The detritus, which, while quartz predominates, is still largely 

 composed of feldspar and mica particles, can not have been carried to any 

 great distance from its source. Tlie quartzite, however, which forms the 

 uppermost horizon of the formations, represents, of course, a more thoroughly 

 assorted detritus, and it is very interesting in this connection to note the 

 extraordinary persistency of tliis horizon. 



At those places in which the Quartz-slate is in contact with the Cherty- 

 limestoue formation, or outcrops of it are not distant, a considerable por- 

 tion of the detritus is often derived from this formation, and close to the 

 contact the chert\' detritus sometimes becomes predominant, when we have 

 true basal conglomerates or recomposed rocks. In the higher horizons of 

 the Quartz-slate tlie clierty detritus is usually sparse or absent. The pres- 

 ence of red jasper fragments in certain of the basal conglomerates suggests 

 the former j)resence of an earlier Iron-bearing formation. 



SaiiiiiKir//. — The Quartz-slate member is given this name because of its 

 slaty character, and of the fact that quartz is its prominent constituent. 



In geographical extent it is the most c(jntinuous belt of the Penokee 

 range, extending fi'om tlie westernmost exposures of the series nearly to 

 Gogebic lake. 



The thickness of the greater ])ortion of the belt varies from 300 to 4< 

 feet, but its maximum thickness is as great as 800 feet. 



Petrographically the Quartz-slate has man}' varieties; the most persist- 

 ent of these is a vitreous (juartzite, which is found at the uppermost horizon 

 of the member. 



The quartz-slates are always fragmental. Their induration is due (1) 

 to the enlargement of quai-tz fragments, rarely to enlargement of the 

 feldspars; and (2) to tlie alteration of the feldspars to other minerals, the 

 most abundant of which are biotite, chlorite, and quartz. 



