NORTH AMERICAN GEOLOGY. 



215 



uary, and was probably coutemporaneous with that of the Sierra Nevada. 

 It is thouglit that the great lava flow of the Ii^orthwest commenced at 

 the beginning of the Pliocene, as it lies upon the eroded edges of the 

 Miocene, and must have continued almost to the present time, the 

 greatest flows being as late as the end of the Pliocene.* 



79. Willis reports on his studies for the Northern Transcontinental 

 Survey of the coal measures on the western slope of the Cascade Mount- 

 ains, in Washington Territory. While the report is in greater part of 

 economic interest only, some general statements are made which form 

 an important contribution to the knowledge of the geology of this dis- 

 trict. The coal measures are in sandstones and carbonaceous shales of 

 Laramie age which aggregate 13,000 feet in thickness. The workable 

 beds are seventeen in number, mostly in the lower 3,000 feet of the 

 measures. The quality varies from poor lignite to coking coal in pro- 

 portion to the mechanical disturbance of the beds. It is locally authra- 

 citic in the vicinity of intrusive dikes. The formation is found to be 

 a brackish water deposit, and by the uplift forming the Cascade Eauge 

 they were flexed over axes having a general north and south trend. Ero- 

 sion, volcanic overflow, and drift have obscured the limits of the forma- 

 tion, and it is now only recognized in detached areas. The following 

 table summarizes the conclusions in regard to the equivalency of the 

 several formations flanking the Cascade Mountains : t 



Eussell, in a very interesting and finely illustrated paper on the ex- 

 isting glaciers of the United States, describes with some detail his own 

 studies of those near Mono Lake, California. Nine glaciers are found 

 in the southern rim of that basin, and a number of others occur on the 

 opposite side of Sierra Nevada, all being between latitude 36° 30' and 

 38° north, at an altitude of approximately 11,500 feet. They occur in 

 amphitheaters on the northern side of elevated peaks, and are quite 

 small, the largest, on Mount Lyell, being nearly a mile in length, and 

 a little more than a mile in width. The ice is ribboned in structures, 

 has dirt bands, crevasses, moraines, and other features found in larger 

 glaciers. Their former wide extent is indicated by great moraines and 



* Am. Jour. Sci., iii, vol. 32, pp. 167-181. 



t Tenth Census Report on Mining Industries, pp. 759-781. 



