I9I5.] THE GEOPHYSICAL STANDPOINT. 303 



past uplift, or subsistence, or horizontal thrust, or cracking under 

 tension, or cracking produced by shear, or microscopic yielding in 

 detail such as produces schistosity for example, or some other form 

 of past yielding to stress-differences. The physicist studying the 

 earth must take this overwhelming mass of evidence into account 

 and must conclude that the earth habitually yields slowly to the 

 stress-differences brought to bear upon it. Please note that I do 

 not assert that the stress-differences are all due to gravity. 



I propose now to state what are in my opinion probably the lines 

 of least resistance to future progress in studying the earth from the 

 physical standpoint. I propose to outline what I believe to be the 

 most effective methods of attack, and to indicate some of the conclu- 

 sions which will probably be reached. I am led to this procedure 

 by two considerations. First, I find it possible to state certain of 

 my opinions as to the net outcome of past investigations most clearly 

 in that form — and time presses. Second, I indulge the hope that 

 such an outline which is frankly an expression of judgment based on 

 evidence much too weak and conflicting to be proof, may possibly 

 kindle the imagination of some man or men, and so lead to vig- 

 orous attacks upon the problem and to future progress. 



In attacking the problems of the earth one should assume at the 

 outset that the phenomena exhibited are very complicated, that they 

 are probably due to various simultaneous actions, and that the 

 various actions are probably closely interlocked, modifying each 

 other, though some are probably primary in importance and others 

 secondary. Hence the most effective method of attack is probably 

 one which includes a general correlation of apparently widely sep- 

 arated ideas and facts gathered from physicists, engineers, geol- 

 ogists, chemists, etc., and at the same time includes intensive attacks 

 in detail on one after the other of single features of the problems 

 which arise and an intensive working out of the possible conse- 

 quences of said features. 



It should be recognized at the outset that no observed behavior 

 of the earth clearly warrants the assumption that the material of 

 which it is composed differs radically in any way from that acces- 

 ible at the surface. It should be assumed, therefore, that through- 

 out the earth the materials are a mixture differing from the mixture 



