GEOLOGY OF THE LONG LAKE QUADRANGLE 477 



The rock is exceedingly variable, much more so than is the 

 anorthosite. All the varieties grade into one another, so that any 

 separation in mapping is an arbitrary matter, necessitating the 

 drawing of boundary lines where none exist. Yet the extreme 

 variations are so unlike -the normal rock as to require separate rock 

 names, and must be given a place upon the map, even at the cost 

 of arbitrary boundaries. 



Normal syenite. This is a green to grayish green rock when 

 fresh, with a rapid color change on exposure to the weather, assum- 

 ing a yellow-brown tinge and then becoming a rusty brown, the 

 normal color on exposed surfaces. Over most of the district the 

 weathered crust is not thick, and in any opening in the rock the 

 normal green is quickly reached. It is a highly feldspathic rock, 

 only second to the anorthosite in this respect, but carries from 

 lo per cent to 20 per cent of other minerals, quartz, pyroxenes, 

 hornblende and magnetite. Quartz never wholly fails, though not 

 rising to large proportions in the normal rock. The pyroxenes are 

 peculiar, and characteristic. The feldspar (microperthite) is of an 

 entirely different nature from that of the anorthosite, is never 

 iridescent, and does not show twinning striations on cleavage faces. 



The original rock was not so universally porphyritic as was the 

 anorthosite, was seldom coarsely so, and even where coarsest was 

 not comparable to the anorthosite in that respect. Hence, though 

 the two rocks have experienced substantially equivalent meta- 

 morphism, the syenite has mostly been mashed by the process, 

 uncrushed feldspar cores being very few and very small in com- 

 parison with the anorthosite. They are generally present in the 

 normal rock however. For this reason the syenite has a more 

 gneissoid look, and an appearance of greater metamorphism which 

 is deceptive. 



The variations of the rock are in two main directions. In the 

 one case the dark colored nainerals increase in quantit}^ at the 

 expense of the feldspar, garnet appears, and quartz diminishes 

 and disappears. The syenite passes into a monzonite and ulti- 

 mately into a shonkinite. The rock also becomes more even 

 grained and gneissoid, as does the anorthosite in its similar vari- 

 ation. The more basic varieties have the dark minerals equaling 

 or exceeding the feldspar in quantity and so strongly imitate 

 the gabbro gneisses of the anorthosite border that they are exceed- 

 ingly difficult to distinguish. In fact distinctly intermediate 



