39 
The hornblende is in smaller amount than the mica. It is pleochroic, 
# — yellow, b — bright green, c — dark dull green. 
The alteration of the hornblende is to chlorite and epidote. The triclinic 
felspars are the predominant feature in this rock, and are twinned according 
to the albite and, secondarily, according to the Carlsbad law. 
One felspar, of relatively well-defined outline, included a later generation 
of a dark-green hornblende. 
All these felspars are more or less kaolinized. 
The measurements made of the extinction angles in this slice are given 
in the following table. Before discussing them, I must note here that the 
thin slices numbered 446, 447, 448, 451, 452, 453 are all of the same 
general character, but are mostly more altered or more decomposed than 
No. 445. They need no further special mention, unless to say that they are 
quartz-mica-diorites. 
Table of Extinction Angles. 
No. of Slice. 
P 
Zone P K 
Zone P 3Vr 
M 
(001) 
(001) (100) 
(001) (010) 
(010) 
445 
1° 10' 
15° 0' 
• 
+ 6° 16' 
446 
2° 45' 
... 
7° 5' 
447 
0° 0' 
16° 22' 
6° 32' 
448 
0° 0' 
12' 24' 
7° 15' 
449 
0° 0' 
10° 39' 
... 
450* 
0° 32' 
... 
7° 5' 
453 
0° 0' 
... 
454 
20° 59' 
• • • 
455 
1° 0' 
... 
* Granophyric quartz-diorite. 
In the above are given the minima on P and the maxima on the other 
planes. In No. 445 I was enabled to determine on a well-marked section, 
near to M (010), that the sign was positive, and therefore that the felspar 
belonged to the oligoclase group. 
The general similarity of the readings in each series shows that the 
plagioclase in all the samples belongs to the same group ; and, as I have 
already said, one at least is oligoclase. 
According to Dr. Hintze* the composition of oligoclase is from Ab6 Anl 
to Ab3 Anl ; and of andesine from Ab3 Anl to Abl Anl ; and he quotes the 
more recent statements of Schuster that in oligoclase the extinction angle 
on P (001) is from + 1° 04' to 2° 45', and on M (010) from + 4° 36' to 
-f- 11° 59'. On andesine he states it to be from — 0° 35' to — 5° 10' on 
P (001). 
According to Levy and La Croixf the maximum extinction is in 
the plane perpendicular to M, or, as I read the observation, in the zone P K, 
4-12° for oligoclase and 4- 21° for andesine. 
The above table shows that these felspars form a group which lies between 
oligoclase and andesine, covering the last of the former and the first of the 
latter. 
It seemed desirable to obtain some further data to control, if possible, 
the optical results of my examination of the felspars of these rocks, as 
exhibited in the thin slices. Mr. Henry C. Jenkins, the Government 
* C. Hintze, Handbuch der Mineralogie, Leipzig, 1897, pp. 1474, 1496. 
t Les Mineraux des Roches, 1888, pp. 205, 207- 
