Car/ties in Rocks exposed to I Hah Pressures, 085 



case. Again, the form given to the specimens under expe- 

 riment is such as to enable the first beginnings o£ yielding 



to be detected. There is in every case a polished flat 

 surface, unsupported, in a small central area. Examination 

 of this by a bright reflected light affords a most sensitive 



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test of distortion. And, in addition to this test, examination 

 with the lens or microscope may readily be applied to this 

 polished surface and minute cracks detected in an incipient 

 stage. When a cavity in one hemisphere is provided rapid 

 yielding reveals itself in the debris which spalls off and 

 accumulates in this cavity. 



The importance of securing the means of sensitive obser- 

 vation arises from the fact that in Nature time for the 

 accumulation and development of effects exists in a far 

 greater degree than can ever prevail in the laboratory. Our 

 only chance of detecting such effects must be such conditions 

 of sensitiveness as will enable their first beginnings to be 

 observed. 



Observation soon revealed the fact that while rapid 



yielding of the material to the external hydrostatic stress 



was shown by the accumulation of debris in the cavity formed 



in one of the hemispheres, less intense stress might show a 



distinct effect in the optical distortion of the flat surface 



vis-a-vis to it or by the development of cracks in the same. 



It is very improbable that such signs of yielding could arise 



without ultimate breakdown of all resistance and closing of 



the cavity. And when we further bear in mind that the 



effects of raised temperature which must prevail in Nature 



are here absent, we seem justified in treating such stresses as 



produce these first signs of yielding as probably exceeding 



those which in Nature would result in the closing of cavities. 



The use of a flat thin washer between the hemispheres was 



dictated by this consideration. And although it possessed 



the disadvantage of slightly disturbing the truth of the 

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spherical form in the specimens as prepared lor these expe- 

 riments, it increased the number of observations possible 

 with the available number of specimens. AVar-eonilitions 

 had put an end to all possibility of obtaining others. 



The rocks dealt with were of four markedly differ cut kinds, 

 and all of widespread importance: — Granite, basalt, obsidian, 

 and Solenhofen lithographic limestone. The material has 

 necessarily to be fine-grained. The granite was from Selb, 

 Bavaria: the basalt from Jungfernstein, Siebengebirge ; the 

 obsidian from Iceland : and the lithographic limestone from 

 Solenhofen, Bavaria. The resistance to crushing of cubes 

 of the granite, basalt, and lithographic slate was made the 



