ANDESIT10 DIKE ROCKS. 309 



andesites (1498, 1500, 1533, 1540, 1541); another, the same with a little 

 mica (1535); two, hornblende-pyroxene-andesite-porphyry (1538, 1539); 

 two, homblende-andesite with possibly a little pyroxene (1548, 1549); 

 eight, hornblende-mica-andesites (1532, 1534, 1556, 1543, 1545, 1557, 

 1558, 1547); and one, dacite (1546). The majority of them are thus seen 

 to be hornblende-pyroxene-andesites and hornblende-mica-andesites, while 

 more basic and more acid varieties are comparatively scarce. 



The pyroxene-andesite (1499) which is a dike in Mount Chittenden 

 is holocrystalline with typical pilotaxitic microstructure, produced by 

 microlites of plagioclase, augite, altered hypersthene, and magnetite. The 

 very small phenocrysts are labradorite, augite, and hypersthene, with 

 opaque paramorphs after hornblende, which sometimes have a central 

 portion composed of pyroxene and magnetite. The hornblende-pyroxene- 

 andesites are holocrystalline, with microstructures ranging from pilotaxitic 

 (1498, 1500) in the dike rock at the summit of Mount Chittenden, and in 

 a dike on the south side of Sylvan Pass (1540), which rocks have rather 

 large phenocrysts of hornblende, through those that are slightly micropoi- 

 kilitic (1533, 1535, 1541), to those that are so plainly microcrystalline as to 

 be classed as andesite-porphyry (1538, 1539). In these latter varieties 

 quartz is recognizable as a component of the groundmass. It is probably 

 present also in the rocks with incipient micropoikilitic structure. 



The phenocrysts are very small and abundant, and vary somewhat 

 in their relative proportions, pyroxene being scarce in some cases, and 

 biotite being present in small amount in one instance (1535). The 

 feldspar, which is abundant, is labradorite, but probably of different com- 

 positions, which is indicated by the optical properties. Hornblende is 

 greenish brown, with no black border, but in a few cases with a narrow 

 border of magnetite grains. Iiitergrowtli with augite was observed in one 

 instance. Augite and hypersthene have the characteristics common to 

 these minerals in all the andesitic rocks of this region. They are subordi- 

 nate to hornblende, and generally in smaller crystals. Magnetite and color- 

 less apatite in small crystals may be classed with the phenocrysts. The 

 rock forming a narrow dike on the north side of Middle Creek, halfway 

 down its course, has a cpiite different character (1548, 1549). It consists 

 almost wholly of groundmass, with only a few scattered phenocrysts of 

 greenish-brown hornblende and lime-soda feldspar. The groundmass con- 



