STURGEON QUARTZITE IN FELOH MOUNTAIN EANGE. 401 



uncomplicated by subordinate folds. It is probably safe to conclude, in 

 view of the uncertainties, tliat the average thickness of the formation is not 

 less than 450 feet, and may be considerably more. In a preliminary paper 

 on the district,^ written before the field notes were fully analyzed, I have 

 placed the thickness of the quartzite at about 700 feet; but this figure is 

 probably too large. 



PETROGRAPHICAL CHARACTERS. 



The Sturgeon formation includes a few very closely related rock 

 varieties, of which quartzite furnishes the great majority of the exposures. 

 The quartzites are usually light gray in color, and break with a coarsely 

 granular or glassy fracture. To the eye quartz is often the onl}^ recogniza- 

 ble constituent in the body of the rock, although the numerous joint and 

 shearing planes shimmer with little silvery plates of muscovite. Occasion- 

 ally a weathered surface is dotted with minute specks of an opaque pinkish 

 substance, which leads one to suspect the presence of feldspar. Chlorite 

 also is now and then visible in the darker varieties. 



The quartzites are almost uniformly massive, except for the secondary 

 fractures above mentioned. At scattered localities, however, a faint color- 

 banding, due to the presence of layers of a pinkish hue, which are inde- 

 pendent of the secondary fractures, seems to indicate the original stratifica- 

 tion. The color bands are generally only vaguely defined ; occasionally, 

 however, they are numerous and sharp. 



Closely associated with the massive quartzites are sheared quartzites, or 

 micaceous quartz-schists. These rocks are merely varieties of the quartzite 

 in which secondary shearing planes, with their attendant growths of new 

 muscovite, are more abundant than usual. The shearing surfaces almost 

 invariably intersect, with the result that the new structure tends toward the 

 linear-parallel type, and is often as similar in appearance as it is in origin 

 to the structure already described in connection with the sheared granites. 



In a locality already referred to, on the south bank of the Sturgeon, 

 jn sec. 36, T. 42 N., R. 29 W., where the Sturgeon formation is in visible 

 contact with the Archean, the quartzite is underlain by a considerable 

 thickness of very fissile muscovite-biotite-gneiss, which incloses rather 

 sparingly obscure pebbles of granite and quartz. This gneiss, which no 



' Relations of the Lower Menominee and Lower Marquette series in Michigan (Preliminary) : Am. 

 Jour. Sci., Vol. XLVII, 1894, p. 217. 

 MON XXXVI 26 



