GEOLOGY OF THE REMSEN QUADRANGLE 



13 



The most abundant constitutent is feldspar, which shows a 

 range of from 65 to 80%, while the average amount present is a 

 little over 70%. Of the feldspars, the anorthoclase and the micrd- 

 perthite are most common, sometimes the one and sometimes the 

 other predominating. The anorthoclase is recognized by the very 

 minute multiple (microcline) twinning bands, often indistinct but 

 presenting a sort of moire effect. Occasionally the multiple twin- 

 ning is more distinct and then the feldspar may indeed be micro- 

 cline. Microperthitic intergrowths especially in the larger feldspars 

 are often beautifully shown. Acid plagioclase is a constant con- 

 stitutent of the syenite, though always subordinate in amount. 

 It is mostly oligoclase with low extinction angles although at times 

 some of it ranges over to andesine. 



The second most abundant mineral is quartz, which makes up 

 from 10 to 25% of the rock or on the average a little over 15%. 

 The quartz grains vary greatly in shape and size and sometimes 

 they are larger than the feldspars. The quartz percentage is often 

 high enough to make the rock a quartz syenite at times approaching 

 granite. The quartz, as well as the feldspar, in nearly all cases 

 appears to have been entirely recrystallized. 



Among the dark colored minerals hornblende is a very constant 

 constituent. It appears to be common hornblende with its charac- 

 teristic cleavage and pleochroism from light to dark green. Often 

 it may be seen partly or completely changed to chlorite. The maxi- 

 mum amount of hornblende present is about 10 or 12%. 



Biotite mica is also always present in scattering flakes, never 

 making up more than 5% of the rock. 



The pyroxenes — augite and enstatite (or bronzite) are fre- 

 quently present but always in small quantity. 



Magnetite — never above 5% — is a constant constitutent. 

 The development of leucoxene borders around the magnetite may 

 often be seen. 



Many very small prismatic crystals of apatite, generally as in- 

 clusions in feldspar or quartz, occur scattered through the rock. 



Besides the minerals mentioned occasional small crystals of 

 zircon and titanite are present. All of the dark colored constitu- 

 ents may occur in the same specimen but together they never make 

 up more than 20% of the rock. 



Syenite-Grenville complex. The rocks here described occupy 

 approximately one half of the Precambric area. Rocks of many 

 different kinds have been included under this head and, both because 

 of the small scale of the map and the scarcity of exposures, it has 

 not been possible to separate these rocks upon the geological map. 



