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sections. It is a microperthite made up of intimately intergrown 
potash-felspar and plagioclase; sometimes a rather high power 
is required to make out the structure. Between crossed nicols 
the sections exhibit a mosaic-like or reticular appearance, yet 
both felspars show a marked tendency to form veinlets parallel 
to 6 (010), or, in other specimens of the rock, perpendicular 
to b (010). The potash felspar is а microcline without cross- 
hatching but showing a quite irregular twin-structure, one in- 
dividual of the twin forming numerous dots or stains within 
the other; in some felspar-sections only one microcline-indivi- 
dual is seen. In the more decomposed varieties of the rock 
the potash-felspar is turbid and has the appearance of ortho- 
clase. — The plagioclase of the microperthite shows a fine 
twin-lamellation according to the albite-law; when examined 
after Becxe’s method it proves in all positions less highly re- 
fringent than quartz. The examination of a number of sections 
showing symmetrical extinction of the twin-lamelle gave for the 
plagioclase a maximum angle of extinction amounting to 15° 
or 16°, while the microcline in the same sections gave values 
varying from 0° to 9°. As to the microcline the maximum 
angle of extinction in symmetrical sections was found to be 
about 19°, and the extinction-angle of the plagioclase of the 
same sections was 9° or 11°. This proves that the plagioclase 
is almost pure albite. The felspar of this rock, then, is a 
perthitic microcline-albite. 
The relative proportion of microcline and albite varies 
somewhat, yet as a rule the albite is predominant. The mar- 
ginal zones of the felspar consist sometimes of pure albite, 
less frequently crystals are found consisting entirely of this 
mineral. 
The greenish colour of the fresh felspar is due to minute 
prisms or slender needles of »girine imbedded in the felspar. 
In some cases the felspar also encloses small patches of arf- 
vedsonite. In the red varieties of the rock the felspar contains 
