abstracts: geology 193 



GEOLOGY. — Coastal glaciers of Prince William Sound and Kenai 

 Peninsula, Alaska. U. S. Grant and D. F. Higgins. U. S. 

 Geological Survej' Bulletin 526. Pp. 75, vAth. maps, views, and 

 sections. 1913. 

 The history of the glaciation of the coastal region of Prince William 

 Sound and Kenai Peninsula includes the formation of an extensive 

 Pleistocene ice sheet, which extended to the sea and reached upward 

 to the mountains. Since this period of maximum glaciation there has 

 been a marked decrease in the extent of the ice-covered areas, until 

 now only valley glaciers reach the sea. This withdrawal of the ice was 

 probably punctuated by temporary advances, and the present is only 

 an epoch in the long history since the maximum Pleistocene glaciation. 

 Earth movements have also played a part, as yet little known, in this 

 history; changes on Prince William Sound are still taking place, and 

 since the maximum period of glaciation there has been a considerable 

 sinking of the coast Hne in much of the area and between Resurrection 

 and Nuka bays there are nmnerous drowned cirques. The glaciers 

 here studied do not give uniform evidence as to a general retreat or a 

 general advance \vithin the last half century; some are evidently in a 

 period of retreat and others in a period of advance, and the general 

 balance between retreat and advance can not be accurately determined 

 by data now at hand. A. H. Brooks. 



GEOLOGY. — Geology and ore deposits near Lake City, Colorado. John 

 DuER Irving and Rowland Bancroft. U. S. Geological Survey 

 Bulletin 478. Pp. 128, with maps, views, and sections. 1911. 



Lake City is in southwestern Colorado and in the heart of the San 

 Juan Mountains. These consist chiefly of volcanic rocks, with intru- 

 sive igneous masses, which now cover an irregular area of more than 

 3000 square miles. The eruptions occurred during Tertiary time, when 

 many kinds of lavas were poured out, building up a huge volcanic pla- 

 teau. There were also long intervals of quiet, during which erosion 

 changed the topography of the volcanic pile. 



In general the Lake City lodes may be regarded mineralogically as 

 the outer or northeasterly edge of the heavily mineralized area of the 

 San Juan. The lodes occur at a slightly older geologic horizon than in 

 the adjacent districts and are definitely characterized by formation at 

 moderate depths. They are fissure veins, formed partty through the 

 replacement of shattered and sheeted zones in the country rock and 

 mainly through the filling of open spaces. They average between 500 



