264 E. C. ANDREWS. 



gravity. The stream must therefore flow round a corner 

 as opportunity offers in seeking the lines of least resistance. 



7. Shearing of rigid and partly plastic bodies is one phase 

 of differential motion ; another phase is found in the violent 

 eddying, the undertow, the splashes and the shore waves 

 of such mobile masses as water. 



Facts of observation gleaned in the Calif omian Sierras. 

 — The observations briefly mentioned here were made in 

 company with Dr. G. K. Gilbert and Mr. W. D. Johnson in 

 August and September 1908, and are specially instructive 

 in any discussion as to the analogies existent between 

 glacial and ordinary stream action. 



All of the following observations were made in areas of 

 recent intense glaciation. The ice masses of the present 

 day are mere glacierets. The departed ice masses evi- 

 dently attained a thickness of at least from one thousand 

 to two thousand feet, and ice polish and glacial striae are 

 abundant. 



L. In the Evolution and San Joaquin valleys quarrying 

 by ice had been an extremely common occurrence. Fre- 

 quently the ice impact had been of such nature that a rock 

 block had been quarried across the dominant joint struc- 

 tures. Ice abrasion had been set up instantly at such 

 points, but in nearly every case the direction of the basal 

 ice had been altered locally, the striae on the quarried sur- 

 face varying in direction with those which apparently had 

 passed over its site just before the quarrying took place. 



In proportion as the downstream slope of the hole or 

 cavity thus produced by the quarrying approached the 

 vertical, so in that proportion had that slope suffered less 

 from abrasion by the downward moving mass. If its slope 

 were very gentle downstream, then the ice striations were 

 almost equally pronounced here as on the flat or gently 

 reversed stoss-seit slopes. 



