273 
line-crystals. They are of a fresh reddish appearance and de- 
veloped in thick tabular crystals which are often Carlsbad twins. 
The thickness of the tables, as a rule, is less than 0'5 centi- 
meter while the length may exceed 2 centimeters. They con- 
sist of a soda-orthoclase sometimes stained with irregular 
patches or shreds of a clouded albite. In sections parallel to 
(010) the soda-orthoclase shows an angle of extinction amount- 
ing to about + 11°, while in the albite areas the corresponding 
angle is about 20°. In the same sections the soda-orthoclase 
upon closer examination sometimes shows an indistinct, broadly 
veined structure at an angle of 72° with (001) corresponding 
to the ordinary perthitic structure. The felspar phenocrysts 
often enclose a great number of small felspar laths of the same 
kind as those of the ground mass. 
The original presence of a third kind of phenocrysts is 
indicated by dull greenish-black stains, less than 0'5 centimeter 
in diameter. They consist of an aggregate of green biotite 
scales, which are sometimes partially altered to chlorite. Rem- 
nants of a colourless augite have sometimes been found enclosed 
within these aggregates. The biotite aggregates, therefore, have 
probably originated from magmatic resorption of augite pheno- 
crysts. 
The prevailing constituent of the ground mass is an alkali- 
felspar which appears in laths of very irregular outlines. There 
is no tendency to parallel arrangement of the laths, but some- 
times radiating groups are found. The length of the laths is 
0:2—0:3 millimeter. Their refractive index is a little lower than 
that of Canada balsam. Many of them show twin lamellation 
with extinction angles indicating albite, others are heteroge- 
neous: the border zone consists of albite while the nucleus 
apparently is a soda bearing orthoclase. Exceptionally an ex- 
tremely fine and indistinct cross-hatched twin structure is ob- 
served. 
Besides felspar the ground mass contains the following 
XXXVIII. 18 
