Ixxvi PROCEEDINGS OF THE GEOLOGICAL society. [May 1902, 
effects of the intrusion of a plutonic magma under the influence of 
the stresses involved in mountain-building have recently been 
discussed by Dr. Weinschenk,' who has suggested that the presence 
of such minerals as epidote and zoisite in some of the central — 
gneisses of the Alps may be due to crystallization under these 
conditions, or, as he terms it, to piezo-crystallization. ; 
I have now to refer to what is generally termed dynamo- 
metamorphism. The idea may be said to have originated with 
the mechanical theory of cleavage. This theory was definitely 
established by the observations of Bauer (1846) and Sharpe (1847), 
and by the experiments of Sorby (1853), Tyndall (1856), and 
Daubrée (1861). Sharpe and Darwin connected foliation with 
cleavage; but Lossen (1867) was, I believe, the first to introduce the 
idea that holocrystalline schists may be produced by the deformation 
of rocks under the influence of earth-stresses. Lossen employed the 
term dislocation-metamorphism, which has since almost 
entirely disappeared in favour of Rosenbusch’s term dynamo- 
metamorphism. Now.it must always be remembered that 
both these terms are somewhat ill-chosen to express the views of 
those who are mainly responsible for the development of the idea. 
They emphasize only one of the two great physical agents involved 
in the production of crystalline schists, and convey the impression 
that heat is excluded. Yet in Lossen’s paper on the crystalline 
schists of the Taunus the action of heat in the presence of water 
is expressly invoked, the mechanical theory of heat is referred to, 
and it is pointed out that if work is done on rocks the temperature 
must be raised. Again, Prof. J. Lehmann, who has perhaps done 
more than anyone else to extend the idea, has pointed out that 
the deformations usually take place under plutonic conditions, 
and therefore at temperatures above those which prevail at the 
surface. 
The point that I am endeavouring to emphasize can be well 
illustrated by a reference to the recent experimental researches of 
Messrs. Adams & Nicolson on the flow of marble.* These authors 
have definitely proved that, when marble is deformed at ordinary 
temperatures by differential pressures exceeding the elastic limit of 
the rock, the flow is due partly to a crushing of the individual — 
constituents and partly to a change in their forms brought about by 
1 ¢Mémoire sur le Dynamométamorphisme & la Piézocrystallisation 
Comptes-rendus du VIII¢™* Congrés Géol. Internat. (1900) p. 326. 
2 Phil. Trans. Roy. Soc. vol. excv (1901) A, pp. 363-401. 
