ABSTRACTS 105 



inents or granules, whose optic axes were in the plane of pressure, but 

 would not change the forms of the crystal fragments not thus oriented. 

 The cr3'stal fragments having their optic axes in the plane of pressure 

 would be changed in shape, as in the experiments cited above, by 

 movements along gliding planes. The stresses in glaciers are mani- 

 festly in various directions, and owing to the flow of the ice, inequali- 

 ties of the rock surfaces beneath, etc., must be exceedingly complex. 

 As the crystal fragments of which glaciers are composed are without 

 orderly arrangement it is evident that some of them will be properly 

 oriented to be deformed by a differential movement of their parts 

 along gliding planes, by pressure acting in any direction. Under 

 diverse stresses the resultant motion would be in the direction of 

 least resistance. That is, the granular ice would behave like a plastic 

 solid. The yielding of glacial ice to pressure, we conceive, is due to 

 movements along gliding planes in the granules of which it is com- 

 posed. If this process is continued the granules will evidently be 

 destroyed by being divided along the planes on which movement 

 occurs, but the resulting subdivisions, or "plates" perhaps we may 

 term them, are reunited probably by the process of regelation. The 

 plates, which after uniting have the same orientation, would form new 

 granules. The longer this process is continued, or, in other words, the 

 farther a glacier flows the greater the chances that a large number of 

 "plates" will come together with similar orientation and the larger 

 will be the resulting granules. 



Under the hypothesis here suggested the granules of glacial ice are 

 considered as an inheritance from the granules in the ice formed by 

 partial melting and refreezing of neve snow. The granules at first are 

 minute, but under the pressure of ice at higher levels they yield along 

 gliding planes, and a resultant motion similar to that of plastic solids 

 under like conditions is initiated. The granules are destroyed by 

 this process, but progressive motion leads to the union of a constantly 

 increasing number of the plates into which they are divided, and the 

 growth of larger granules. 



JVork of the U. S. Geological Survey in the Sierra Nevada. By H. 

 W. Turner. 



To the north of the Fortieth Parallel Mr. Diller has published one 

 folio on the Lassen Peak area, and has a very extensive series of notes 

 which are as yet unpublished. The gold deposits of the Sierra 



