May 29, 1890] 



NATURE 



05 



mical action following from the simple bringing into close 

 contact of molecules. In the union of gases, when they 

 are condensed by platinum-black, and even in the 

 light rubbing of a safety-match on the match-box, we 

 have illustrations of such phenomena. 



Spring has shown that, when powdered metals are 

 mixed together and subjected to great pressures, union 

 takes place between them, and alloys are formed. When 

 drj- anhydrous salts are similarly treated, double decom- 

 position takes place, and new compounds are formed. 



Prof. Thorpe has shown that dry anhydrous salts may 

 be made to react with one another by being simply rubbed 

 together in a mortar ; and both Mr. Hallock and Prof. 

 Spring are agreed as to the intensification of action which 

 occurs when rubbing or sliding movements — attended 

 with necessary multiplication of points of contact in 

 compressed bodies— takes place. 



Lastly, it may be pointed out that Spring has recently 

 shown time to be a very important factor in such changes, 

 by allowing slow diffusion to take place at the surfaces of 

 contact. 



That the rocks known as "crystalline schists and 

 gneisses" have had their peculiar characters produced 

 by " internal differential movements," resulting from 

 "enormous irregular pressures," was clearly recognized 

 by Poulett-Scrope, Darwin, Naumann, and Sharpe long 

 before the researches of physicists and chemists had 

 supplied us with the explanation of the phenomena. 

 Modern petrography has confirmed and illustrated these 

 conclusions, enabling us to study the actual stages of 

 the processes of change by which, through the reaction 

 of the constituent minerals of a rock under pressure, the 

 whole mass resolves itself into a completely different 

 mineral-aggregate. The labours of Lossen in the Hartz, 

 of J. Lehmann in Saxony, and of H. Reusch in Norway, 

 have been of especial value in establishing these import- 

 ant conclusions. 



As an illustration of this kind of action, we cannot, 

 perhaps, do better than take the case of a rock (gabbro) 

 consisting of three somewhat unstable constituents (see 

 Fig. 2), labradorite (a), pyroxene (b), and olivine (c). In 



the rock from which the figure has been taken, there are 

 clear evidences of its having been subjected to deforming 

 stresses. Where the olivine, the least stable of the three 

 minerals, is in contact with the labradorite, the silicates 



NO. 1074, VOL. 42] 



of which they are composed have reacted upon one 

 another. The result is seen in the formation of a zone 

 between them, consisting of entirely new minerals — a 

 pyroxene (d), and a hornblende (e). Similar changes, 

 but not so strongly marked, are seen to be in progress 

 between the olivine and the pyroxene, and between the 

 pyroxene and the labradorite. 



By carefully selecting and studying a series of speci- 

 mens from the same rock-mass, every step in the meta- 

 morphosis of a rock may be followed, from incipient 

 changes like those in the case above illustrated, to the 

 final disappearance of every vestige of the original mineral 

 constituents of the rock, and the substitution of new 

 mineral species. 



XII. When internal strains and differential movements 

 affect a mass, which is at the same time undergoing re- 

 crystallization, the forms and relations of the crystalline 

 particles that build up the new rock may be greatly modi- 

 fied by the action of the 7nechanical forces. 



That perfect rest is a condition upon which well- 

 developed crystallization depends, is a fact too well 

 known to need dwelling upon here. That very small 

 mechanical causes, such as the presence of foreign bodies, 

 or the existence of rough surfaces, may determine the 

 size and position of crystals in a solidifying mass, is also 

 a fact familiar to every chemist. By stirring or similar 

 movements carried on within a crystallizing mass, granu- 

 lation, or the formation of a number of small imperfect 

 crystals, rather than of large and well-developed ones, 

 is brought about ; as in the well-known Pattison's process 

 for desilverizing lead. 



The evidence of perfectly similar actions having taken 

 place in crystallizing rock- masses is everywhere con- 

 spicuous ; and the results are the same, whether the 

 crystallization occurred in a mass passing from a fluid to 

 a solid state, or in a mass which remained solid during 

 the whole process of recrystallization. 



There are two structures which are especially exhibited 

 by rocks that have been subjected to dynamo-meta- 

 morphism which seem clearly to have been produced by 

 such causes ; these are the structures known as \h^granu- ■ 

 litic and \.h.Q foliated. 



ThQ granulitic structure, which is so well exhibited by 

 the rocks called " granulites," is characterized by the 

 crystals assuming the form of granules, having rtiore or 

 less rounded outlines, and lying in every position ; so 

 that, under the polariscope, the mass resembles a fine 

 mosaic. I have shown that well crystalline rock-masses 

 (gabbros), when forced in a molten state through great 

 fissures, assume on their edges, where much friction must 

 have occurred, this granulitic habit, which is sometimes 

 exhibited in a very striking manner (Quart. Journ. Geol. 

 Soc, xlii. 76, &c.). 



The foliated structure, so characteristic of schists and 

 gneisses, consists in the separation along nearly parallel 

 planes of leaf-like patches (folia) of the several mineral 

 constituents of the crystallizing mass. Poulett-Scrope, 

 from his study of the viscous lavas of Ponza, Lipari, and 

 Hungary, and Darwin, from his study of the similar lavas 

 of Ascension, were able to show that these rocks, under 

 similar conditions, often assume a foliated structure. 

 Perfectly granitic rock-masses, like the syenite of the 

 Plauenschen Grund and the granitite of Aberdeen, some- 

 times exhibit on their margins a distinctly foliated 

 structure. 



It is worthy of notice that both the granulated and the 

 foliated structures are produced in recrystallizing masses 

 that are subjected to internal strains and differential 

 movements. They are equally produced when the mass 

 has been a liquid which has slowly passed into the solid 

 state by the process of crystallization ; and when by the 

 processes we have already considered the mass, retaining 

 its solidity, has undergone internal molecular rearrange- 

 ment and recrystallization. 



