STRENGTH OF THE EARTH'S CRUST. 27 



Dr. J. C. Branner : I would like to ask Mr. Gilbert whether he has considered 

 this subject in connection with the subject of glaciation, and whether he believes the 

 weight of ice has anything to do, or much to do. with the northward depression of this 

 country during the glacial epoch. 



Dr. A. C. Lawson : I would like to ask "Mr. Gilbert whether he included the 

 greater inqualities or the less inequalities. 



Mr. Gilbert : I will speak first in reference to the matter of shear, referred to by 

 Professor Stevenson. The generalization, based on many observations, that the 

 material of the earth's crust, under suitable conditions of pressure and confinement, 

 yields to shearing stresses by flowing, finds its exception near the surface, for there 

 the conditions of confinement do not compel flow, but permit fracture ; and it may 

 be added that the result is affected also by differences in the strength and texture of 

 various rocks. But at a great depth, the rock subjected to shearing strain is held 

 closely in its place and cannot part asunder, and the result is a diffused shear, or flow. 

 I conceive that in a general way the phenomena of fracture are quite superficial, be- 

 longing to a tract extending five to ten miles downward from the surface, and that 

 the phenomena affecting the larger problems of terrestrial physics are phenomena of 

 viscous flow. 



With reference to Dr. Lawson's question, which possibly I do not fully understand, 

 I would say that I believe a broad observational basis underlies the general propo- 

 sition that the ocean beds are heavier than the material of the continents. The data 

 have been ably discussed by Pratt, Fisher, and Faye, and the mathematical researches 

 of George Darwin appear to me to demonstrate, not, indeed, his conclusion that the 

 earth is immensely rigid, but the fallacy of his postulate that the earth is homoge- 

 neous as to density. Moreover, as he himself points out, we have a very decided inti- 

 mation, in the grouping or bunching of land masses on one side of the earth and of 

 the ocean on the other, that the distribution of terrestrial densities is not symmetric. 

 If it were symmetric, the center of mass would be the center of figure, and the oceanic 

 waters would be drawn as much to one side as to the other. 



Dr. Branner refers to the bending down of the earth's crust by the weight of the 

 great ice sheet. I regard that hypothesis as most valuable, and one that will stimulate 

 investigation. It is too early yet to accept it or reject it. I may say that it is my 

 own working hypothesis, but I see the opportunity to gather an immense mass of 

 material pertaining to the subject, and until that material has been gathered it will 

 be unwise for us to tie ourselves to any one theory. 



The substance of the next paper read is contained in the following 

 abstract : 



BOULDER BELTS DISTINGUISHED FROM BOULDER TRAINS — THEIR ORIGIN 



AND SIGNIFICANCE. 

 BY T. C. CHAMBERLIN. 



[Abstract.} 



For obvious reasons, boulders were among the first phenomena of the drift to attract 

 attention, and occupied a large share of consideration in the earlier days of investiga- 

 tion of glacial phenomena. In recent years attention has been more largely directed 



