GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 39 



anorthosite-gabbro are visible across the whole rock ledge, the 

 country rock (in places) at the contacts plainly showing the effects 

 of the heat of intrusion. Toward the interior of the mass the 

 diabase exhibits an excellent diabase texture, white feldspar laths 

 up to one-quarter of an inch long standing out in a finer grained, 

 dark, bluish gray ground mass. A thin section from the interior of 

 the dike shows the following mineral percentages : labradorite 44 ; 

 augite (pale reddish brown) 18; biotite 8; chlorite 18; magnetite (or 

 ilmenite) 11; and pyrite i. Due to more rapid chilling, the dike 

 borders are very fine grained to even glassy. About 100 yards 

 northeast at the lake edge, this dike again outcrops. It here also 

 shows branches, the greatest visible width being 27 feet. The dike 

 strikes south 40° west. 



The other dike also lies on the shore of Long lake and less than 

 one-half of a mile northeast of the bridge. Its main body is 

 exposed for 35 feet with strike south 40° west and a width up to 

 6 feet, though one wall only is shown. A very small tongue branches 

 off this portion. It is fairly evident that this dike formerly extended 

 at least 200 feet farther northward but has been eroded away along 

 the shore leaving the vertical wall of the country rock distinct. 

 What appears to be a separate dike here 6 inches to i foot wide and 

 30 to 40 feet long is quite certainly only a branch of the larger 

 one now eroded away toward the north. This branch dike is 

 distinctly brecciated due to faulting. In thin section the rock from 

 the 6 foot wide dike shows practically the same mineral composition 

 as the larger one farther northward already described. 



The perfect alignment of the strikes of these two dikes suggests 

 the possibility that they are really portions of a single intrusive 

 which is largely concealed under the lake, but of course they may 

 be entirely separate. Also the fact that these dikes lie in or close to, 

 and parallel to, the zone of fracture (fault) which has determined 

 the position of the Long lake depression, would suggest that the 

 ■ diabase w^as intruded along this fault zone but, if so, there was 

 renewed faulting in the same zone because one of the dikes is 

 brecciated. 



ROCK STRUCTURES 

 Foliation 



Except the diabase and certain pegmatites, all the Precambrian 

 rocks of the quadrangle show more or less foliation. As a rule, the 

 gabbro masses are devoid of foliation except around the borders 



