GEOLOGICAL STRUCTURE OF HIGH ROCK DISTRICT. 99 



parallel with this occur along the top of the ridge and have been worked 

 at various points. In all some 35 or 40 openings have been made on this 

 property. 



Geological structure. — The hills are made up of quartzite, pyroxenite, 

 and gneiss in belts whose direction corresponds with that of the ridge. 

 The apatite occurs in veins or pockets in the pyroxenite. The quartzite 

 occurs in beds standing in a nearly vertical position and with strike 

 parallel to the general direction of the ridge. 



The pyroxenite is not distinctly banded, though occasionally parallel 

 lines, which have sometimes been taken to represent lines of stratifica- 

 tion, can be traced through it. 



Stratified and massive gneisses are reported* as often seen bordering 

 the hills in the apatite region, but they were not made the subject of 

 study at High Rock. 



The rocks all dip at high angles (nearly vertical), and are cut in vari- 

 ous directions by small dikes. 



At several places on the hill bosses of feldspar rock appear protruding 

 through the quartzites and pyroxenites, and expanding at the surface. 

 An instance of this is seen near the summit in front of the office. The 

 feldspar is coarsely crystallized, of a lilac color, and is associated with a 

 considerable amount of augite. 



In most of the openings, the apatite is associated with a reticulated 

 feldspar rock, consisting of lumps of coarse feldspar (and a variable 

 amount of quartz), separated from each other by thin anastomosing layers 

 of green augite and a small amount of fine grained feldspar. This rock, 

 which it is the chief purpose of this paper to discuss, is closely associated 

 with the coarse feldspathic rock above mentioned, and evidently belongs 

 to the same rock body. In places it has a distinctly striped gneissoid 

 appearance. In general it is composed chiefly of an alkali feldspar and 

 augite, with a comparatively small amount of quartz, thus presenting the 

 mineral composition of an augite-syenite. As the rock in all its phases 

 shows more or less evidence of dynamic action, it is properly to be re- 

 garded as a gneiss, and may therefore be designated as an augite-syenite- 

 gneiss. 



Subdivisions of the Syenite-gneiss. 



The term syenite-gneiss is here used to include a peculiar assemblage 

 of rocks occurring at High Rock and neighboring mines, among which is 

 the so-called " leopard rock " or "concretionary veinstone " of the Cana 

 dian geologists. The rock presents three distinct phases, which for con- 



* i'enrose : Bulletin 4C, U. S. Geol. Survey, p. 26. 



