290 CALIFORNIA ACADEMY OF SCIENCES, [Proc. 3D Ser. 



some of which, sticking fast in the glacier, were quite lifted 

 up from the bed of the cirque, while others were touching 

 or resting on it. He thinks it probable that as the tempera- 

 ture around the glaciers constantly varies about the freezing 

 point, the incessant freezing and thawing of the water in 

 the cracks in the rock may split it, and the glaciers may do 

 the work of transportation for the fragments thus broken 

 loose. On examining the interior of an empty cirque, we 

 observe that a bursting, not a scooping-out of the rocks 

 has taken place." 



Dr. Becker^ describes a mode of erosion, suggested by 

 observations in Alaska, which embodies practically the same 

 idea as that just described. He writes: — 



In considering the glaciers at various points it occurred to me that disinte- 

 gration must be extremely rapid along the edge of the ice, for the tempera- 

 ture of the ice being zero centigrade, the number of days in the year must be 

 great upon which the adjoining rock is chilled below the freezing point at 

 night and raised above the melting point during the day, and such alternations 

 form a most efficacious means of breaking up solid rock. ***** 

 It appears to me that all along the edge of a glacier and along its \\€\€ field 

 as well, frosts must be more frequent than, ceteris paribus, at a distance from 

 them, and that disintegration must be correspondingly rapid. The U-shape 

 assumed by glacial gorges and the cirques at the neve basins may be in part 

 explicable in this way. 



yohnsoii' s Thco7y. — Mr. Willard Johnson, in a paper 

 read before the Geological Society of Washington, had pre- 

 viously presented nearly the same theory for the formation 

 of cirques and steps in glaciated canyons, although this was 

 not known to Dr. Becker at the time of his writino- the 

 above ; but Mr. Johnson considers that this mode of erosion 

 produces results greater in amount than should perhaps be 

 ascribed to it. The following is an outline of the main fea- 

 tures of the theory, which he kindly wrote out and placed at 

 my disposal. The steps of the cyclopean stairway found 

 in some of the glaciated canyons are regarded as the work 

 of ice, and the formation of these steps is the first point 

 described. 



1 Reconnaissauce of the Gold Fields of Southern Alaska. Kighteeiith Annual Report 

 U. S. Geol. Surv., Part III, 189S, p. 60. 



