MEANING OF ROCK FLOW AGE. 749 



the class of rocks in which the process has taken place on the most exten- 

 sive scale. It will be remembered that slates, schists, and gneisses are 

 rocks in which many of the mineral particles show parallel orientation and 

 in which the major portion of the mineral particles now show slight 

 or no strain effect. (PI. XI, C.) It is evident that these are rocks 

 which have nearly perfectly accommodated themselves to the deformation 

 through which they have passed. The accommodation, as already 

 explained, is accomplished by continuous solution and deposition, or by 

 continuous recrystallization. While the adjustment during - deformation at 

 any moment was nearly as complete as though the rock were a magma, 

 and while it may nowhere show even a microscopic opening, it is evident 

 that the flowage is wholly different from that of a liquid. At no time was 

 the rock a liquid; on the contrary, it was at all times almost wholly a 

 crystallized solid. At no time was more than an almost inappreciable 

 fraction of it in liquid form — that is, dissolved in water — yet at all times it 

 was adjusting itself by means of the small percentage of water contained 

 in the capillary and subcapillary openings, this being the chief agent of 

 deformation working through solution and deposition, or recrystallization. 

 In order that such a continuous process shall Lj adequate to explain rock 

 flowage, it is necessary only that it shall be sufficiently rapid +o keep pace 

 with the deformation. Probably one's first thought is that it is not possible 

 that the process can be sufficiently rapid to account for the phenomena. 

 However, the experiments of Barus on the solution of glass give us a basis 

 upon which we can make a quantitative calculation. 



Barus has shown a that a temperature of 185° G- is critical so far as the 

 solution of glass by water is concerned. At temperatures lower than this 

 the rate of solution by water is very slow. However, at temperatures of 

 185° C. and above, solution of the silicates of glass goes on with astonishing 

 rapidity. Barus dissolved,- in capillary tubes, a sufficient amount of glass 

 to cause an apparent contraction of volume of the water amounting to 13 

 per cent of the water present in 42 minutes, and 18 per cent in an hour. 



This illustrates the fact that the activity of water at a very moderate 

 temperature is amazing and one need not be surprised at its potency in the 

 alteration of rocks deep below the surface of the earth. Temperatures 

 higher than 185° C. exist at moderate depth, and therefore it is reasonable 



a Barus, C, The compressibility of liquids: Bull. U. 8. Geol. Survey No. 92, 1892, pp. 78-84. 



