8 6 Van I Use — Metamorp/iism of Rocks an d Bock Flowage. 



to the zone of recrystallization and then decreases for a certain 

 depth. No conjecture is made as to how far down this decrease 

 continues but it is believed to be probable that the decrease; 

 continues at least as deep as the zone in which the schists formed 

 by recrystallization develop. 



Meaning of Rock Flowage. 



I have previously maintained that the rocks within the scope 

 of our observation which have been deformed at considerable 

 depths were deformed by rock flowage. However, I made no 

 attempt to explain what actually occurred during the process. 



I shall take as a typical example of rocks which have been 

 deformed in the zone of flow those laminated crystalline schists 

 the mineral particles of which now show slight or no strain ; 

 for it is evident that these are the rocks which have nearly per- 

 fectly accommodated themselves to the deformation through 

 which they have passed. The accommodation, as already ex- 

 plained, is accomplished by continuous solution and deposi- 

 tion, 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 nowhere shows 

 more than a microscopic space, 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 crystalline solid. At no time was more than an almost 

 inapplicable fraction of it in a liquid form — that is, dissolved in 

 water — yet at all times it was adjusting itself by means of this 

 small percentage of water contained in the capillary and sub- 

 capillary spaces, this being the medium of solution and recr} T s- 

 tallization. In order that such a continuous process shall be 

 adequate to explain rock flowage, it is necessary only that it 

 shall be sufficiently rapid to keep pace with the deformation. 

 One's first thought is probably that it is not possible that the 

 process can be sufficiently rapid to account for the phenomena. 

 However, the experiments of Barus upon the solution of glass 

 give us a basis upon which we can make a quantitative calcu- 

 lation. 



Barus has shown that a temperature of 180° C. is critical so 

 far as the solution of glass by water is concerned. At temper- 

 atures lower than this the rate of solution by water is very 

 slow. However, at temperatures of 185° C. and above, solu- 

 tion and crystallization of the silicates of glass go on with 

 astonishing rapidity. In Barus' experiment water dissolved a 

 sufficient amount of glass and deposited the material as crystal- 

 lized minerals to cause an apparent contraction of volume of 

 the water amounting to 13 per cent of the water present in 

 the capillary tubes in 42 minutes and 18 per cent in an hour. 

 This shows that solution continued during the later stages of 



