May 29, 1890] 



NATURE 



lOI 



Popocatapetl. 



In vol. xli. of Nature, (p, 592) you state : " Despatches from 

 Mexico state that observations show that the height of the active 

 volcano Popocatapetl has decreased by 3000 feet since the last 

 measurement." This despatch, which was forwarded from Prof. 

 Heilprin's party now in Mexico, would seem to indicate that 

 there has recently been an actual loss of height in Popocatapetl ; 

 whereas Prof. Heilprin's object was to indicate that the observa- 

 tions'hitherto accepted are inaccurate, 



Edmund J. de Valois. 



295*Adelphi Street, Brooklyn, N.Y., 

 May 16. 



CHEMICAL CHANGES IN ROCKS UNDER 



MECHANICAL STRESSES^ 

 A FTER pointing out that his object was to inquire how 

 -^*- far the experimental researches of chemists and phy- 

 sicists are capable of affording a satisfactory explanation 

 of the phenomena observed when the rocks of the earth's 

 crust are studied microscopically in thin sections, the 

 lecturer proceeded to give a r'estimJ of the experimental 

 investigations of Daubrde, Bunsen, Sorby, Thorpe, Spring, 

 Guthrie, Fouqu^, Michel- Ldvy, and other chemists, who 

 have devoted their attention to the action of pressure 

 in influencing chemical affinity. The evidence that the 

 deeper-seated rock-masses of the globe, and those con- 

 stituting mountain-chains, have been subject to enormous 

 pressures was then indicated ; and the difference between 

 the statical pressures arising from a great weight of 

 superincumbent rocks, and the dynamical pressures 

 resulting in actual movements within the earth's crust, 

 was insisted upon. The chemical and physical principles 

 which have been established by direct experiment, and 

 which, at the same time, appear to be illustrated by the 

 observations that have been made during recent years 

 upon the minute structure of rocks, and of the minerals 

 composing them, were stated in the following series of 

 propositions : — 



I. I>t all those cases in which crystallization is accom- 

 panied by contraction, the tendency of pressure is to 

 promote the change from an amorphous to a crystalline 

 condition. 



Spring has shown that under a pressure of 6000 

 atmospheres plastic or amorphous sulphur, having a 

 density of 1-95, passes into rhombic, crystallized sulphur, 

 having a density of 205. 



The mixtures of silicates which constitute the igneous 

 rocks of the earth's crust all undergo contraction in passing 

 from the amorphous (vitreous) to the crystalline condition. 

 This is easily proved by comparing the specific gravities 

 of more or less crystalline rock-masses with that of the 

 glasses formed by their artificial fusion. The experiments 

 of Delesse, Deville, Cossa, and others have shown that 

 mixtures of the silicates of alumina and the alkalies with 

 over 70 per cent, of silica, must undergo a contraction to 

 the extent of ^V of their bulk in passing from a glassy 

 to a highly crystalline state (granite). Mixtures of the 

 silicates of alumina, magnesia, iron, lime, and the alkalies 

 with less than 50 per cent, of silica, in passing from a 

 vitreous state to a perfectly crystalline one (gabbro), must 

 undergo a reduction in bulk equal to I. 



It may fairly be anticipated, therefore, that great 

 pressure would tend to promote the crystallization of the 

 mixtures of silicates composing most of the rocks of our 

 globe, or to prevent their assuming the glassy state ; and 

 a great body of geological facts tends to support this 

 conclusion. It must not, of course, be lost sight of that 

 slow consolidation is also favourable to the process of 

 crystallization, and rocks being extremely bad conductors, 

 the process of cooling in great rock-masses is excessively 

 slow. It is often difficult therefore to discriminate 



' " The Evidence afforded by Petrographical Research of the Occurrence of 

 Chemical Change under Great Pressure." A Lecture delivered before the 

 Chemical Society, March 20, 1890, by Prof. J. W. Judd, F.R.S. 



NO. 1074, VOL. 4^^ 



between the effects that must be referred to slowness of 

 cooling, and those which may be safely considered to 

 result from pressure. 



As long ago as 1846, Charles Darwin showed that the 

 andesitic lavas of the Cordillera of South America are 

 associated with perfectly crystalline rock, true granites, 

 made up of precisely the same minerals. The identity of 

 the minerals in the plutonic rocks and the lavas respec- 

 tively was demonstrated by the careful studies of Darwin 

 himself, and of Prof W. H. Miller, of Cambridge, long 

 before the method of studying rocks in thin sections had 

 been invented. Quite recently Prof. A. Stelzner, employ- 

 ing the modern methods of research, has been able to 

 completely confirm the interesting results arrived at by 

 Darwin and Miller, and to show that a perfect gradation 

 can be traced between the highly crystalline " Anden- 

 granites," and the more or less glassy lavas (andesites) 

 which are so closely associated with them. 



In 1874 I was able to show that in the Western Isles of 

 Scotland there occurred masses of perfectly crystalline 

 (granitic) rock, identified by Zirkel as true gabbros and 

 granites, which can be traced passing by the most 

 insensible gradations into natural glasses (" tachylytes " 

 and " obsidians ") (Quart. Journ. Geol. Soc, xxx., 1874, 

 233-48), and the truth of these conclusions has been 

 fully established by the more recent researches of Dr. A. 

 Geikie (Trans. Roy. Soc. Edinb., 1888, 122-24, H5- 

 50), In 1876 I further showed that the diorites and 

 quartz-diorites of Hungary and Transylvania pass in- 

 sensibly into the ordinary lavas of the district, which have 

 the same ultimate chemical composition, and the same 

 mineralogical constitution (Quart. Journ. Geol, Soc, xxxii., 

 1876, 292). In 1885, Messrs. Arnold, Hague, and 

 J. P, Iddings, of the United States Geological Survey, 

 established precisely similar conclusions by the study of 

 rocks in the Nevada district (Bull. U.S. Geol. Surv., No. 

 17, 1885) ; and Signor B. Lotti,of the Italian Geological 

 Survey, in the following year proved the same to be true 

 in the case of the rocks of Elba. 



In all these cases it is seen that the masses which have 

 been most deeply seated, and thus subjected to the 

 greatest statical pressures, are those which have under- 

 gone the most perfect crystallization. It must of course 

 be remembered that in these cases the other cause tending 

 to the development of crystalline structure comes into 

 play — namely, slowness of cooling. The ordinary materials 

 of igneous rocks are such bad conductors of heat, that 

 enormous periods of time must elapse before the deeply 

 seated portions of igneous rock-masses can become 

 solidified. 



The potent influence of this extreme slowness of cooling 

 in bringing about the crystalline structure in mohen 

 masses of silicates has been well illustrated by the splendid 

 researches on rock-synthesis by MM. Fouqud and A. 

 Michel-Ldvy. They have shown that the secret of making 

 a particular mineral crystallize out of such a mass consists 

 in finding out the temperature of fusion of the mineral, 

 and in maintaining the molten mass for a long period just 

 below this temperature. In the excessively slow cooling 

 of deeply seated rock-masses, the materials must be kept 

 successively and for long periods at temperatures a 

 little below the fusion-points of each of their mineral 

 constituents. 



But while the influence of slow cooling in producing 

 the crystalline structure in rocks is unquestionably very 

 great, the effect of pressure in promoting crystallization 

 can scarcely be doubted. We have no proof, indeed, that 

 the holocrystalline or perfectly granitic structure of rocks 

 can ever be produced except under these conditions of 

 extreme pressure. 



II. Crystallized minerals, developed in a magma under 

 pressure, may lose their stability and be dissolved by the 

 same magma when the pressure is removed. 



The very remarkable researches of Fouqu^ and Michel- 



