182 F. D. ADAMS EXPERIMENT IN GEOLOGY 



izes and the loss from the various grains is, of course, proportionate to 

 their surface area ; the smaller grains, having a surface area relatively 

 great as compared with their mass, have a tendency to decrease in size 

 and disappear, while the larger grains, owing to their greater mass, con- 

 dense this vapor on their surface and thus increase in size under the 

 operation law of surface tension. ^^ 



The process is essentially the same as that which takes place when a 

 very fine-grained precipitate of, for instance, barium sulphate is kept 

 warm for some time before filtering, the grain of the precipitate thus 

 coarsening by a process of solution and deposition. This coarsening of 

 finely divided silicates by submitting them to a high temperature when 

 in a state of very fine powder is now, I am informed by Dr. F. E. Wright, 

 regularly employed at the Geophysical Laboratory in Washington for 

 the purpose of obtaining from such fine-grained aggregates crystals suffi- 

 ciently large for purposes of crystallographic measurement or for the 

 determination of their optical properties. The process in the case of 

 many minerals goes forward quite rapidly. 



]^ow this fact, experimentally determined, has probably a very impor- 

 tant bearing on the question of the origin of the metamorphic rocks in 

 general, and especially of those crystalline schists which present the 

 mosaic structure to which reference has been made above; for the ma- 

 terials out of which they have been developed were fine in grain and crys- 

 tallized not only under great pressure, but when deeply buried and there- 

 fore at a high temperature. In many cases they may have been subjected 

 to long-continued heat after the parallelism of constituents had been de- 

 veloped in them by movements under pressure, in which case this coarsen- 

 ing of grain with the final development of a mosaic structure, amounting 

 practically to a recrystallization of the mass, would result. 



This is a new principle in metamorphism, but one v^hich is probably 

 of wide-reaching significance. 



It may be noted in this connection that any malleable metal, when 

 made to flow by rolling or hammering, is deformed by the elongation of 

 its constituent grains through movements on their gliding planes, the 

 hammered or rolled metal thus taking on a structure identical with that 

 in the marble deformed by pressure in the experiments mentioned above. 

 If the metal is then heated and allowed to cool slowly, this fibrous struc- 

 ture completely disappears and is replaced by a typical mosaic structure 

 identical with that ordinarily seen in natural marbles. Under this treat- 

 ment a complete recrystallization of the metal takes place in the space of 

 a few minutes and without the point of fusion being even approached. 



See also Justus Roth : AHgemeine Geologie, Bd. iii, 1, p. 154. 



