GEOLOGY OF THE LONG LAKE QUADRANGLE 505 



This pyroxene is highly exceptional in its lime-magnesia ratio, so 

 much so that it is unsafe to assume, as was hoped might be done, 

 that it is representative of the pyroxenes of these rocks. Since how- 

 ever this assumption involves less uncertainty than the assumption 

 that some other pyroxene may be representative, or the assumption 

 than no pyroxene is representative, it is made use of in the following 

 calculations. It may well be that the pyroxenes formed in highly 

 metamorphosed sediments tend to show a different lime-magnesia 

 ratio from those of igneous rocks, in correspondence with the well 

 known differences in this ratio exhibited by the two classes of rocks. 



This especial pyroxene is of rather dark green color, as evinced 

 also by the iron percentage shown in the analysis. But the pyrox- 

 enes of these Grenville gneisses are by no means uniform in this 

 regard, being often white, or light green. It is however thought 

 that this is simply due to variations of comparatively small range 

 in the iron content, sufficiently small to form a matter of slight 

 importance in the composition of the whole rock. 



Microscopic analysis of the section of the rock by Rosiwal's 

 method gave the result indicated in the following table. Only the 



Units by 

 Units measured Sp. Gr. weight % weight 



Quartz 1506x2.65 = 3991 = 55.50 



Feldspar 664x2.6 = 1726 = 24.01 



Pyroxene 379 x 3 .3 = 1251 = 17.40 



Titanite 51x3.5 = 178 = 2.47 



Zircon 10x4.5 = 45 = 0.62 



Total 2610 7iQt too. 00 



five minerals mentioned were present in the slide and the two latter 

 in but slight quantity. The larger part of the feldspar showed 

 plagioclase twinning with maximum extinction angles of 12 , in- 

 dicating either an acid andesin, about Ab 2 An x , or else albite, in all 

 probability the former. There was also a small amount of un- 

 twinned feldspar which was perhaps orthoclase. As a check on the 

 calculation and an aid in making more certain the character of the 

 feldspar, Professor Morley determined the silica and alkalis in the 

 rock, as follows: Si0 2 , 80.89%; Na 2 0, 1.81%; K 2 0, 0.44%. 



Then the composition of the rock was calculated, first determin- 

 ing the pyroxene from its known composition, using the remainder 

 of the alkalis for the orthoclase and albite determination, and the 

 silica residue forming the quartz. The feldspar deficiency on this 

 basis made it certain that the feldspar was not albite, hence enough 



