84 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 crystal 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 



