Vol. 4] Osmont. — Geological Section of Coast Ranges. 



45 



between the trace of the plane of the optic axes and (010). This 

 value on the n B curve corresponds to an acid labradorite. 



Some of the rock was crushed and cleavage flakes of the feld- 

 spars selected. On examination under the microscope certain of 

 these sections parallel to P, and showing' albite lamallae which 

 extinguish symmetrically, gave the extinction angle 5°. They 

 also showed the emergence of a bisectrix very inclined to the sec- 

 tion. If this angle is negative, it corresponds to a composition of 

 Ah. An,, according to Schuster. Sections showing no twinning 

 lamellae and rhombic in form gave the following angles. 10°, 

 10°, 9y 2 ° and 6°, and extinguished in the acute angle of the 

 rhomb. Hence, the angle is negative. Ab, An, corresponds 

 to 5° 10', while Ab, An 3 gives 17° 40'. Hence the determination 

 points to a composition between Ab, An, and Ab, An 2 . 



In order to determine the specific gravity of the feldspars 

 the crushed rock was put through an 80-mesh screen and caught 

 on a 100-mesh. Thoidet's solution was used. At 2.652 about 

 two-thirds of the feldspars remained suspended, though they all 

 sank at 2.645. The temperature was 15%° C. On concentrating 

 the solution the heavier feldspars became suspended at 2.675. 

 The average specific gravity given by Rosenbusch-Iddings for 

 andesine is 2.65, and of labradorite 2.69, hence this checks xevy 

 well with the optical determinations. It also shows that quartz 

 and orthoelase are absent. 



The biotite presents no novel features. It is strongly pleo- 

 chroic, the color for c being dark reddish brown, I) dark reddish 

 brown, and a straw yellow to yellowish brown. The absorption 

 is C = b > 3. The biotite occurs in irregular plates with frayed 

 edges, and the cleavage lamellae frequently are bent, due to 

 shearing in the rock. It is usually altered around the edges to 

 green chlorite. Magnetite occurs sparingly as inclusions, and 

 occasionally titanite. Apatite is abundant, both included in the 

 biotite and in the feldspar. A small amount of green hornblende 

 was observed in one slide. 



Quart z-Biotite Diorite. — Microscopically this rock is seen to 

 be a coarse grained holocrystalline rock, made up principally of 

 white feldspar and a dark green mica, the feldspar forming about 

 two-thirds of the bulk of the rock. The ferro-magnesian minerals 



