GENERAL GEOLOGY 23 



pleochroism in brown and green. The plagioclase is 

 zonal and is a complexly doubly-twinned labradorite. 



A compact, fine-grained, dark, normal kersantite, with 

 biotite as the only colored constituent, cuts the granite near 

 Glacier Station. Another dike (38), of a normal diabase 

 full of calcite spots, occurs at the same station, and a third 

 dike of diabase (51), a foot wide, at the Summit Station. 



YAKUTAT BAY 



In Yakutat Bay most of the rocks seen in situ belong to 

 the Vancouver or Yakutat Series, described in another 

 chapter. Mr. Gilbert in crossing the moraine of the great 

 Hubbard Glacier at the head of the bay, found among its 

 boulders, besides the very abundant black slate and sand- 

 stone of the Yakutat series, a variety of crystalline rocks 

 probably brought down by the glacier from the higher 

 peaks of the St. Elias chain. As little is known of the 

 nature of the rocks constituting this crystalline axis, it 

 seems worth while to catalogue these rocks. 



Glaucophane-quartz-schist (188). A bluish to greenish, compact 

 rock with several parallel white quartz veins traversing the specimen. 

 Under the microscope the fine granular quartz of which the rock chiefly 

 consists is seen to be full of needles of strongly pleochroic glaucophane, 

 which gives the rock its prevailing blue color, and of grains of epidote 

 which are in places sufficiently abundant to color the rock greenish. A 

 somewhat similar quartzite was found in boulders in the moraine of 

 Hidden Glacier. 



Amphibolite (190). A very massive, coarse, granular rock, consist- 

 ing principally of green fibrous hornblende with small amounts of 

 interstitial plagioclase and quartz. Possibly a basic segregation in a 

 massive diorite. 



Hornblende-gneiss (192). Blackish contorted gneiss, made up of 

 dark needles of hornblende and plagioclase grains. Considerable 

 rutile in twinned crystals, and a little quartz. 



Epidote-quartz-schist (194). A mottled and banded greenish and 

 white rock consisting chiefly of granular quartz with relatively large 



