86 ALASKA GEOLOGY 



matitic intergrowths, laths of acid plagioclase and a little diopside in 

 grains. 



DIORITE-APLITE 



No. 86, forming a dike twenty feet wide, which stands out as a 

 prominent wall in the shales on the upper slopes of Chichagof Peak, 

 is a very compact grey rock showing minute glassy feldspar pheno- 

 crysts and rather abundant though small pyrite crystals. Under the 

 microscope it was found to consist almost wholly of triclinic feldspar. 

 The phenocrysts, which are sparse, are highly twinned albite crys- 

 tals. There are also occasional pseudomorphs of calcite after what 

 seems to have been hornblende. The groundmass is a network of 

 lath-shaped crystals of oligoclose, in the minute interspaces of which 

 is calcite, and chlorite which may represent original pyroxene, al- 

 though no unaltered pyroxene could be found. Magnetite and pyrite 

 are sparingly present and several apatite needles were noted. 



No. no, from a narrow dike on the shore of West Cove, is similar 

 in character to the foregoing, but contains biotite and small amounts 

 of pyroxene in the feldspathic groundmass. 



These rocks seem best classified as diorite-aplites, occurring as 

 they do in connection with an intrusive mass of dioritic nature. 



DIORITE-PORPHYRITE 



No. 82. Near the summit of Chichagof Peak, on the east side, a 

 large dike, about thirty feet in width and conspicuous by its light 

 color, cuts both the sedimentaries and the diorite-porphyrite intrusive 

 in them (see fig. 18). The rock constituting this dike is a porphyry 

 of rather fine grain with numerous dark green hornblende and snowy 

 plagioclase crystals in a compact greenish groundmass. The feldspars 

 predominate, giving the rock as a whole a light grey color. In thin 

 section it was found to consist of phenocrysts of labradorite and horn- 

 blende in a groundmass of albite, hornblende, colorless pyroxene and 

 accessory magnetite and titanite. 



The labradorite was determined as such by extinctions on the sharply 

 idiomorphic albite-Carlsbad twins. It is abundant, making up perhaps 

 a third of the rock. 



The hornblende is in slender prisms with f ringed-out ends, and is 

 weakly pleochroic, green to brown in pale tints. It is not abundant, 

 and is sometimes wholly altered to chlorite. 



The groundmass is a finely interwoven aggregate of albite laths 

 with faintly green hornblende needles, some or all of which may be 



