Cavities in Rocks exposed to High Pressures. GS7 



The Experiments. 



(I.) Two sphereswere dealt with : one of basalt, the other 

 granite. The hemispheres wore cemented by bard Canada 

 balsam. The pressure was raised little by little to 50 tons 

 read on the manometer. This was maintained constant from 

 December 20th, 1912, till March 4th, 1913, I e. for (58 days. 

 During this time the plunger sank in about ^ mm., or 

 possibly a little more. When pressure was relieved the 

 lower plunger under the elastic recovery of the lead within 

 the mortar protruded about 3 mm., lifting the heavy crushing 

 mortar by this distance. On forcing out the lead cylinders 

 the copper washers showed considerable flowago, where they 

 were pressed against the plungers. The lead cylinders were 

 easily parted, and by gentle warming the hemispheres 

 opened. 



Both basalt and granite had yielded in the same manner. 

 The cavity held a small quantity of powdered rock — 

 considerably more in the granite than in the basalt sphere. 

 The debris in the basalt weighed 0*100, and in the granite 

 0'170 gram. Under the microscope these powders were in- 

 distinguishable from rock powdered — not very finely — in an 

 agate mortar. 



The flat polished rock surfaces covering the cavity had 

 bulged downwards. The amount of bulge in the case of the 

 basalt was estimated to be not more than 0*1 mm. The bulge 

 on the granite was rather more. 



It is difficult to reproduce the appearance of the specimens 

 by photography — but the photos figs. 1 & 2 (PL XVI.) show 

 that the effects are quite conspicuous. The manner in which 

 the cavity in the basalt has been extended is remarkable. 

 The bulge on the flat covering surface faithfully follows the 

 outline shape of the cavity. 



Examination with a strong lens of the flat, bulged, surfaces 

 showed that the distortion was largely due to the relative dis- 

 placement of the individual mineral particles. This points 

 to the risk of error which may obtain when experiments of 

 this nature are carried out on simple minerals only. 



It nay be safely inferred from this result that the pressure 

 of 50 tons prolonged over the duration of the test is con- 

 siderably in excess of what would suffice to close the cavity. 



(II.) As in (T.) granite and basalt spheres were dealt 

 with — fresh specimens being, of course, taken. The granite 

 in this case, although from the same locality, has a felspar 

 which is of a pale pinkish tint, and in that respect seems to 

 differ from the granite of (I.) and as used in the crushing 



