MASS MECHANICAL ACTION. 49 



to barium carbonate and sodium sulphate." In all such chajiges the 

 fmadanieiital principle contrcJljjig is that reactions shall take plage whicli 

 result in sjnaller vojjrmes. Spring 6 found that in the case of dry reactions 

 induced by mechanical action time is a very important factor, the reactions 

 taking- place much more slowly than when compounds are moist and water 

 is an intermediate agent. 



MASS MECHANICAL ACTION. 



Mass mechanical action (a) may permanently strain the rocks without 

 openings, (b) may strain the rocks with rupture and openings, and (c) may 

 close the openings in rocks and produce welding-. 



permanent strain without openings. — In order that permanent strain beyond 

 the elastic limit without openings may take place in the rocks it is nec- 

 essary that deformation shall occur while the rocks are under a sufficient 

 pressure in all directions to hold the molecules so close together that the 

 molecular attraction is effective. This will be true only where the pressure 

 is greater in all directions than the crushing strength of the rocks. It is 

 well illustrated by Adams and Nicolson's experiment on the deformation 

 of marble while under pressure in all directions." The molecules were 

 held close to one another, and the deformed marble retained considerable 

 strength. 



Later we shall see that the process of readjustment may be mechanical 

 or chemical or partly each. When the process is mechanical the mineral 

 particles are usually granulated — that is, finely fractured. When the 

 process is chemical the particles are reci'ystallized. Also the process of 

 readjustment may be accomplished by any combination of granulation and 

 recrystallization (See pp. 737-748.) Under natural conditions, in order 

 that the pressure in all directions shall be greater than the crushing strength 

 of a rock, it is necessary that it be in the zone of flowage for that rock. 



permanent strain with openings. — When the rocks are strained beyond the elastic 

 limit and the pressure is not greater in all directions than the crushing 

 strength of the rocks, rupture and openings are produced. The ruptures 

 may be regular or irregular. The regular ruptures may be of great extent 



a Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895, 

 p. 390. 



6 Spring, op. cit., p. 322. 



c Adams, F. D., and Nieolson, J. T., An experimental investigation into the flow of marble: 

 Philos. Trans. Koyal Soc. London, ser. a, vol. 195, 1901, pp. 363-401. 



>ION XLVII — 04 i 



