An Unrecognised Agent in the Deformation of Rocks. 393 



but where two grains come together the material has been removed. 

 Instances of the same kind on a larger scale are very common in the 

 limestone conglomerates called the Nagelfluhe," which occurs in 

 Switzerland and in the foot-hills of the Jura. Prof. Daubreef in 

 trying to imitate the conditions of Nature, took two balls of limestone 

 and placed them in a slightly acidulated solution, the upper one 

 exerting at the point of contact a pressure of ten kilograms upon the 

 lower. The result was just the reverse of what occurs in Nature, the 

 carbonate of lime was dissolved from the surface of the spheres and 

 deposited at the point of contact. He then varied the conditions. 

 The two balls were placed one on top of the other as in the previous 

 experiment, but a very small amount of liquid was allowed to drop on 

 them from a cotton thread suspended above them and hanging over 

 the side of a vessel containing the acidulated solution. Directly a 

 drop fell on the spheres, it found its way to the point of contact, and 

 was held there by capillary attraction. Under these circumstances, 

 the limestone at the point of contact was dissolved and the centres 

 of the two spheres slowly approached one another. 



The effect of this on a bed of ordinary granular limestone when 

 subjected to pressure is obvious. If a bar of any material is bent, 

 the outer edge is in a state of tension, and the inner of pressure. 

 If a bed of limestone, then, is subjected to a distortional force in the 

 earth's crust, that part which is under pressure will have the 

 constituent grains tending to dissolve, while that under tension will 

 atford a resting-place for the matter dissolved, and the whole bed 

 will in this way adjust itself till equilibrium is restored. 



The effect of solution on more resistent substances like the natural 

 silicates and quartz, is exactly similar to the carbonates, except that 

 the operation must go on at greater depths than the carbonates. 

 Van Hise has very thoroughly gone into the question, and has 

 shown that in all probability the schistose rocks, in which the 

 silicates and quartz are elongated parallel to the shearing, have 

 assumed this character, not by a melting under enormous tempera- 

 tures of the original material, and recrystallization of the minerals 

 normal to the direction of pressure, but to a gradual solution of the 

 minerals in the positions of pressure and a redisposition of the 

 material in the positions of tension, the whole operation going on at 

 comparatively raoderate depths and temperatures. 



The rate at which this solution goes on is probably very rapid. 

 We have no actual experiments on rocks, but supposing that the 



* Heim, " Mechanismus cler Gebirgsbildung," Basel, 1878, atlas, pi, xiv., Fig. 12 ; 

 see also Mellard Eeade, Geological Magazine, 1895, p. 344. 

 t " Geologie Experimentale," pi. 382. 



