THE INTEEIOR OF THE EARTH. 243 



one of the strongest materials existing in the earth in large quantities. 

 Two-tenths of a ton per square inch is the safe working load used by 

 engineers for good granite. There is abundant evidence from 

 laboratory tests that the so-called yield point on which the engineer 

 bases his estimate of safe working load for a given material is a 

 function of the length of time the load is applied and the delicacy 

 of the test. The longer the time of application and the more refined 

 the test to determine the permanent yield the lower the observed 

 yield point. In the case of the test in progress in the earth the time 

 of application is indefinitely long and the test is extremely refined 

 inasmuch as the minimum rate of yielding which may be detected 

 is exceedingly small. 



If an engineer wishes to know whether a bridge, or foundation, 

 or building, or railroad rail is yielding under stress differences which 

 have been brought to bear upon it he looks for evidence of distress, 

 for rivet heads popped off, scaling from the surface, settling, cracks, 

 or even changes in microscopic structure. The geologists have made 

 very extensive corresponding examinations of the earth. Everywhere 

 they find evidence that the earth has yielded. On the one-fourth of 

 the earth's surface exposed to examination, the land, there is no part 

 for which the evidence does not indicate past uplift, or subsidence, 

 or horizontal thrust, or cracking under tension, or cracking produced 

 by shear, or microscopic yielding in detail such as produces schis- 

 tosity, for example, or some other form of past yielding to stress 

 differences. The physicist studying the earth must take this over- 

 whelming 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 con- 

 clusions 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 vigorous 

 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 vari- 

 ous actions are probably closely interlocked, modifying each other. 



