( 'avities in Rocks exposed to High Pressures. 689 



In the case of sphere B there was no visible change pro- 

 duced in either hemisphere. 



It seems as if we must conclude that at normal tempe- 

 ratures the pressure of 20 tons on the plunger must be near 

 that critical pressure which will just determine the closing 

 of small cavities. This load on the plunger, the area of 

 which is 0*775 square inches, produces a pressure of 

 511,145 lb. per square inch, or 4107 kilos per square cm. The 

 depth of the earth's surface, assuming the crust-rock to 

 possess a density =2*8, at which such a pressure might 

 prevail, would be about ( J miles. The temperature prevailing 

 at this depth might be about 450° C. (Adams, loc. cit.). It 

 would be desirable to repeat the experiment at this tem- 

 perature. It seems probable that this pressure would, under 

 the high-temperature conditions, ultimately close every cavity. 



(IV.) In this experiment the granite and basalt hemi- 

 spheres used in (III-), which showed no visible injury, were 

 re-arranged as before. No Canada balsam was used. The 

 second sphere was composed of a hemisphere of Solenhofen 

 limestone and one of obsidian. No balsam was used. The 

 washers were re-ground. A pressure of 22 tons was applied 

 from March 21st to March 23rd, 1914, i. e. 2 days, and then 

 it was increased to 30 tons and left till March 27th, i. e. 

 4 days more. 



The results were: — A faint raised circular area in the case 

 of the basalt — so faint that it is not easily seen without 

 breathing on the polished surface. The granite reveals no 

 definite effect. 



The Solenhofen limestone shows a quite distinct raised area 

 corresponding to the opening in the washer. A straight-edge 

 applied across the stone showed the slight central elevation. 

 The obsidian showed a similar effect but very much fainter. 



Comparing these results with (II.), we are entitled to 

 conclude that what difference there is was due to the lesser 

 duration of the stress in (IV.). 



(V.). It was now resolved to apply the procedure of (I.) 

 and (II.) to the limestone and obsidian, and to seek for a 

 definite positive result at 30 tons. Two limestone hemi- 

 spheres were placed vis-a-vis. One of these was that which 

 had been used in (IV.), and which exhibited a small but 

 distinct bulge. The other carried the cavity, and had not 

 been previously used. The obsidian sphere was prepared in 

 the same manner, one of the hemispheres being that which 

 was used in (IV.). No balsam was used in either sphere. 



