226 Johnston and Adams — High Pressures on the 



rough parallelism between relative ease of flow and melting 

 point, namely : that a substance can be welded more easily the 

 lower its melting-point ; to this point we shall revert later. 



Experimental Investigations of the Effect of (Non- 

 uniform) COMPRKSSION ON SOLID BLOCKS 



of Minerals and Rocks. 



A large number of papers dealing with the behavior of solid 

 blocks of minerals and rocks when exposed to compression have 

 been published. Most of these are of interest only in connec- 

 tion with the compressive strength* of rocks (e.g., for use as 

 building material) and hence need not even be referred to here; 

 several, however, which have a direct bearing on the question 

 under discussion will be treated briefly. 



The general procedure adopted in these investigations was that 

 suggested originally by Kick. If the material was a crystalline 

 aggregate, it was fashioned into a cylindrical block which was 

 inclosed in a tight-fitting steel tube by heating the latter and 

 shrinking it on to the material. When separate crystals were 

 employed, they were placed in a piece of stout copper tube of 

 suitable size and the interstices tilled up with an embedding 

 material — e. g., sulphur, fusible metal, alum, or paraffin wax — 

 which at the end of the experiment was removed by solution in 

 water or by fusion. In either case the pressure was applied to 

 the two ends of the metal tube, and continued in gradually 

 increasing amount until considerable deformation of the tube 

 and contents ensued. From the mode of experiment it is ob- 

 vious that the compression was never uniform, and became less 

 and less uniform as the strength of the supporting tube became 

 less. This is the main disadvantage of the method, for it is 

 impossible to arrive at more than a very rough approximation 

 in an endeavor to estimate the amount of pressure in any 

 direction ; consequently qualitative results only can be obtained 

 by this means. 



The most extended investigation along this line is one by Frank 

 D. Adams,f who worked with a series of minerals of varying 

 hardness, as follows : selenite (2), rock salt (2*5), iceland spar 

 (3), fluorite (4), apatite (5), diopside (5 # 5), limonite (5-6), ortho- 

 clase (6), magnetite (5*5-6'5), pyrite (6-6 5), quartz (7), garnet 



* The compressive strength of rocks, its variation with the mode of com- 

 pression, and the precise significance (or lack of significance) of such results 

 in relation to the question of the depth of the '• zone of flow " in the earth's 

 crust are ably discussed in two recent papers, one by F. D. Adams (J. Geol- 

 ogy, xx, 97-118, 1912), on the experimental side, the second a mathematical 

 discussion by L. V. King (ibid., 119-138). 



f " An Experimental Investigation into the Action of Differential Pressure 

 on Certain Minerals and Rocks, employing the Process suggested by Profes- 

 sor Kick " (J. Geology, xviii, 489-505, 1910). 



