GEOLOGY AND PALEONTOLOGY 19 



side and grains of epidote and titanite. The hornblende 

 is in prisms, slender and very sharply bounded, showing 

 pinacoids and often terminal planes also. It often encloses 

 feldspar grains. It is generally very fresh, but occasionally 

 shows alteration to chlorite. The feldspar showing albite 

 twinning was proved by extinction and refraction to be 

 oligoclase. The untwinned feldspar seemed to be of the 

 same species, rather than orthoclase, which might be ex- 

 pected to occur in this rock. The diopside is frequently 

 surrounded by chlorite, and has probably furnished most 

 of that mineral, which is quite abundant in parts of the 

 slides, and is the only decomposition product present. A 

 slide cut across the contact with the granite (60) shows at 

 the contact a band, i mm. wide, of apparently pure glass, 

 and the spessartite quite uniformly grows coarser-grained 

 away from the contact. 



The hot waters of the springs, which have a tempera- 

 ture of about 150 F. and contain sulphur and carbonic 

 acid, seem to rise through the granite, which is the near- 

 est outcropping rock. 



Dr. Dall visited Biorka Island, off Sitka Harbor, and 

 found it to consist of a light-colored biotite tonalite, per- 

 haps a phase of the Hot Springs granite. It is cut by a 

 single large dike of rhyolite, which shows a pronounced 

 parting into small columns. The thin section of this rock 

 (51 and 52) shows a well-characterized flow structure, 

 with abundant development of coarse spherulites along 

 the lines of flow and about small scattered phenocrysts of 

 quartz, oligoclase, and Carlsbad twins of orthoclase, the 

 remainder of the groundmass being microgranular. 



GLACIER BAY 



In Glacier Bay a light-colored quartz-diorite or tona- 

 lite was found to be the principal rock on the west side, 

 as already determined by Reid. 1 It is cut by innumerable 



1 Glacier Bay and its Glaciers, i6th Ann. Rep. U. S. Geol. Survey, part I 

 p. 433- l8 96- 



