MEANING OF ROCK FLOWAGE. 319 
In discussing the subject of rock flowage I shall summarize and to a 
certain extent repeat the substance of preceding parts of this paper. 
This seems necessary in order to bring the various ideas which concern 
rock flowage into their proper relations to one another as bearing upon 
that subject. 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 ex- 
plain what actually occurred during the process. The foregoing discus- 
sion explains this to some extent at least. 
I shall take as a typical example of rocks which have been deformed 
in the zone of flow those laminated crystalline schists the mineral parti- 
cles of which now show slight or no strain (plate 19, figure 2); for it is evi- 
dent that these are the rocks which have nearly perfectly accommodated 
themselves to the deformation through which they have passed. The ac- 
commodation, as already explained, is accomplished by continuous solu- 
tion and deposition, or by continuous recrystallization. While the adjust- 
ment during deformation at any moment was nearly as complete as though 
the rock were a magma, and while it nowhere shows even 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 inappreciable 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 per- 
centage of water contained in the capillary and subcapillary spaces, this 
being the medium of solution and recrystallization. In order that such 
a continuous process shall be adequate to explain rock flowage, itis nec- 
essary only that it shall be sufficiently rapid to keep pace with the defor- 
mation. One’s first thought is probably that it is not possible that the 
process can be sufficiently rapid to account for the phenomena. How- 
ever, the experiments of Barus upon the solution of glass give us a basis 
upon which we can make a quantitative calculation. 
Barus} has shown that a temperature of 180° C. 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 tempera- 
tures of 185° C. and above, solution and crystallization of the silicates of 
glass go on with astonishing rapidity. In Barus’ experiment, as already 
seen, water dissolved a sufficient amount of glass and deposited the ma- 
terial as crystallized minerals to cause an apparent contraction of vol- 
ume of the water amounting to 13 per cent of the water present in the 
* Principles of North American pre-Cambrian Geology, by C. R. Van Hise: 16th Ann. Rept. U.S. 
Geol. Survey, pp. 593-595, 636-643. 
+ Compressibility of liquids, by C. Barus: Bull. U. 8. Geol. Survey, no. 92, 1892, pp. 78-84. 
