266 A. C. LANE — GEOLOGIC ACTIVITY OF THE EARTH's GASES. 



tend far enough down into the earth's crust to strike a depth where there 

 are abundant absorbed gases, and where there is a strong tendency to 

 give them off, without first striking a plastic or viscous, either sohd or 

 liquid, zone, through which a crack could no more go than through a 

 stream of honey or the sand of an hour-glass ? 



There are experiments of great importance by Kick '^ which seem to 

 answer in the negative. In seeking and finding with brilliant success 

 definitions of brittleness and hardness more precise than usual, he found 

 that a block of so brittle a substance as salt, which has, however, been 

 lately shown to be much more plastic than quartz or glass, enclosed in a 

 copper tube and subjected to a strong hydrostatic pressure by filling the 

 tube with oil and then screwing in a stopple, could be pressed from 8.1 

 millimeters down to 5.3 millimeters in a hydraulic press without being 

 cracked. Calcite behaved similarly when enclosed in steam.f 



Now, practically all rocks will be crushed by a pressure of 2,000 kilos 

 per square centimeter, a pressure reached at 8,000 meters or five miles 

 beneath the surface of the earth. 



Below this, therefore, rocks might be expected to be highly plastic, 

 and even above this they might perhaps be softened by superheated 

 water, although if this water were already charged up to its capacity with 

 mineral matter, its weakening effect may not be what some have thought. 

 At any rate, long before we reach the region of anything like the heat 

 of many lavas, or have more than barely come to a depth where the 

 shrinkage through loss of heat tends to be more than the radical con- 

 traction, we shall have reached a plastic zone. 



Dwect Evidence as to its Occurrence. — This is certainly a serious difficulty, 

 yet I think not fatal, and before trying to show the possibility of crack- 

 ing far down into the earth, it is well to remember that, whether we can 

 explain it or not, there are good reasons entirely, independent of this 

 theory, to believe that it does so break. 



In the first place, the depth of origin of earthquake shocks, which are 

 generally accepted to be due to some kind of a break (in fact, can hardly 

 be otherwise), have been estimated by noting the angle of emergence and 

 circle of maximum throw,;]] by estimating the index circle of most rapid 

 decrease in intensity of effect,§ and by comparing the times of arrival at 

 different points. || By these methods we find that the source of earth- 



* Dingler's Polytechnic Journal, 1889, p. 448; abstracted in Scientific American. 



t It seems quite possible that the enclosing fluid may have been solidified by pressure in Kick's 

 experiment. 



X Mallet discredited. See Geikie : Text-book of Geology, 1893, pp. 276, 279 ; and Schmidt, Neues 

 Jahrbuch, 1893, vol. i, p. 65, and vol. ii, p. 44. 



§ Button's Report on Charleston Earthquake: U. S. Geol. Survey, 1888, p. 313 ; Geikie: Op. cit., 

 pp. 272 and 279. 



II V. Seebach : Das Mitteldeutsche Erdbeben, von G Milrtz 1872, 1873. 



