GEOLOGY OF THE LONG LAKE QUADRANGLE 461 



whole mass. They are however a much more prominent feature 

 of the thinner, variable beds, than of the large masses. 



All these rocks are very quartzose, but all contain feldspars in 

 respectable amount. There are two main types of the rock min- 

 eralogically. In one a light-colored pyroxene (a white or light 

 green diopside) is a prominent constituent, while mica (usually 

 phlogopite) if present is subordinate; in the other the pyroxene 

 is subordinate or fails, and mica assumes much greater prominence. 

 The pyroxene rocks are much more apt to be granular and 

 weakly resistant to wear, while the mica rocks are comparable to 

 many of the igneous gneisses in resisting power, hence their ten- 

 dency to form hills. Much of the rock strongly resembles quite 

 pure quartzite, but careful inspection always shows a considerable 

 feldspar or pyroxene content. 



The larger number of the Grenville exposures in the district show 

 a quite var3dng set of rocks in comparatively thin layers. The 

 quartz pyroxene gneisses described above constitute an important 

 feature. There are frequent, thin, micaceous bands in which, in 

 addition to the mica, there is increased pyroxene and feldspar, 

 and much diminished quartz, and which would seem to represent 

 thin shale bands. Equally frequent are basic bands of hornblende 

 mica gneiss, with black pyroxene and soda-lime feldspars for the 

 other constituents, which have thus the mineralogy of gabbros, but 

 are distinctly interbanded with the sediments. Rather thin bands 

 of limestone occur frequently, generally quite impure, showing 

 more or less pyroxene, titanite and graphite, grading often into 

 border rocks of black, heavy character and composed chiefly of 

 pyroxene. These limestones are interbanded with, and grade into 

 the quartz pyroxene gneisses, producing all sorts of intermediate 

 rocks, so that the series as a whole seems made up of alternate 

 limestones and sandstone bands, with an occasional thin layer of 

 shale. Exposures do not suffice to determine whether thick lime- 

 stone masses are, or are not present. Ophicalcite was found in one 

 single locality in the Moose creek belt. This general group of rocks 

 is the one represented at the majority of the Grenville exposures 

 of the quadrangle. Next to it in importance is the heavy quartzite 

 group. No sillimanite gneiss w^as encountered, which is surprising, 

 but not infrequently considerable masses of gneiss, both acid and 

 basic, all cut up by quartz veins, and with frequent bands of solid 

 quartz are met, which look sedimentary but are somewhat doubtful 



