286 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D .Sek. 



tributaries. As in the Mono Lake basin, however, the 

 amount of silt from the glaciers carried off into the San 

 Joaquin Valley cannot be estimated. 



^. Rock Basins. 



Among the proofs of great abrasion by ice on rock sur- 

 faces, the existence of basins apparently scooped out of 

 solid rock is perhaps the most striking. In 1859, Sir. A. 

 C. Ramsay first ascribed the origin of these basins to the 

 action of land ice. His views were not at once accepted, 

 and, in fact, so recently as 1882, Reverend A. Irving^ 

 concludes, on mechanical grounds, that the actual excava- 

 tion of lake basins by glacial ice is inadmissible. Such 

 rock basins are abundant in all mountainous glaciated 

 regions. They certainly do not represent original depres- 

 sions of the surface, for the original surface has long since 

 disappeared, and are plainly not due to subsequent oro- 

 graphic movements. At least this is the case with all those 

 referred to in this paper. The conclusion is therefore 

 drawn that their origin is in some manner due to glacial ice, 

 or perhaps in part to the ice streams that at some points dash 

 through crevasses onto the underlying rocks, although such 

 ice streams probably, in most cases, merely erode pot- 

 holes. 



Glacial markings may often be traced down below the 

 water at the upper end of the lake and found emerging at 

 the lower end with the same steady direction as on the sur- 

 rounding rocks. In the glaciated regions of the Sierra Nevada, 

 rock basins are very abundant. These have already been 

 described by Russell and others. The rock in which 

 they have been found is ordinarily granite. Many of 

 these rock basins are at the base of steep slopes, and it is 

 possible in such cases that the great downward pressure of 

 the ice excavated such basins, especially where the rock was 

 much jointed. The location of other rock basins, however, 



1 "On the Mechanics of Glaciers, with Special Reference to Their Supposed Power of 

 Excavation." Quart. Jour. Geol. Soc. London, Vol. XXXIX, 1S83, p. 62. 



