566 THE JOURNAL OF GEOLOGY. 



hundreds of thousands of years, or even millions of years — if 

 physicists allow us so much. 



The objections thus far brought forward, though serious, are 

 by no means unanswerable. But there is one brought forward 

 very recently which we are not yet fully prepared to answer and 

 may possibly prove fatal. 



5. Level of No Strain. — Until recently the interior contraction 

 of the earth was considered only roughly and without analysis. 

 It was seen that the surface was already cool and its temperature 

 fixed while the interior was still hot and cooling ; and therefore 

 that the exterior must be thrust upon itself and be crushed. But 

 the phenomena are really far more complex than at first appears. 

 It is necessary to distinguish between two kinds of contraction 

 to which the interior layers are subjected, viz., radial and circum- 

 ferential. If there were radial contraction only, then undoubt- 

 edly every concentric shell as it descended into smaller space 

 would be crushed together laterally. But there is for all layers, 

 except the surface, also a circumferential contraction, and this 

 would have just the opposite effect, i.e., would tend to stretch 

 instead of crush. Therefore wherever the decrease of space by 

 descent is greater than the circumferential contraction, there will 

 be crush, and where the circumferential contraction is greater 

 than the decrease of space by descent, there will be tension and 

 tendency to crack. There would be no real cracking, only 

 because incipient cracks would be mashed out or rather prevented 

 by superincumbent pressure. Where these two are equal to one 

 another, there will be no strain of any kind. There is a certain 

 depth at which this is the case. It is called the "level of no 

 strain." To Mellard Reade is due the credit of first calling atten- 

 tion to this important principle. 



Let us analyze the principle more closely. It is admitted 

 that at the surface there is no contraction of any kind. It is also 

 calculated that contraction of all kinds cease at depth of 400 

 miles. It is believed farthermore that commencing 400 miles 

 below the surface and coming upward the contraction increases 

 very slowly from zero to a maximum at the depth of 70 miles 



