8 4 
ALASKA GEOLOGY 
together with fairly abundant magnetite. The rock differs materially 
from the preceding alkali-syenite-porphyries, both in structure and in 
the absence of hornblende, but it is nevertheless, though with some 
doubt, classed with them. 
Many other dikes, from which specimens were not col¬ 
lected, appeared to consist of rocks similar to those just 
described, so that the characteristic dike rock of the region 
may be considered an alkali-syenite-porphyry. 
LATITE 
No. 94. On the lower slope of Chichagof Peak, just above the little 
lake, and cutting the lower Stepovak beds (breccias), is a narrow dike 
of a black glassy-looking rock, weathering into spheroidal forms. 
Glassy feldspar phenocrysts are abundant in the aphanitic groundmass. 
Studied in thin section, this rock proved to be wonderfully fresh and 
lava-like in appearance. In a groundmass of brown glass crowded 
with microlites of plagioclase feldspar and augite,. are numerous phen¬ 
ocrysts of labradorite, orthoclase and augite. The smallest feldspar 
microlites seemed to have the same character as the most abundant 
phenocrysts, which were determined as basic labradorite by extinctions 
of complex albite-Carlsbad twins. The phenocrysts contain numerous 
inclusions of glass, which are grouped parallel to the outlines of the 
crystal and are in part altered to chlorite. But the feldspar itself ap¬ 
pears perfectly fresh and unaltered. 
The orthoclase, which is less abundant than the labradorite, offers 
many points of interest. It is in anhedra with subangular and often 
rounded and deeply embayed outlines. Some are quite unaltered; 
more frequently the whole centre of the ciystal is granulated, the grains 
of orthoclase showing wavy extinctions, the border also orthoclase, but 
in a single individual with uniform extinction throughout; again, with 
the same orthoclase border as in the last, the centre is occupied wholly 
by fibrous chlorite, and in a final stage the chlorite occupies the whole 
space. In some crystals the granular orthoclase and the chlorite are 
intermingled with grains of augite, and it appears as if the chlorite 
were derived from the decomposition of the augite. But there must 
also have been replacement of feldspar substance by chlorite in the 
largely or wholly chloritized crystals. The embayed outline of the 
orthoclase crystals suggests that they were the first separation from a 
magma which afterwards attacked them, in part dissolving, in part 
