[02 



NATURE 



[May 29, 1890 



L^vy upon the synthesis of rocks is not less instructive, 

 whether we consider the successes or the failures of their 

 experiments. While able to reproduce by fusion and slow 

 cooling— either from the powdered rocks themselves, or 

 from duly admixed proportions of silica, alumina, iron 

 oxide, and the alkaline earths and alkalies — various kinds 

 of basalts and other basic rocks, all attempts to form 

 certain other rocks, especially those containing quartz, 

 hornblende, and muscovite, failed. The conclusion at 

 which the experimenters arrive — and the correctness of 

 this conclusion it is scarcely possible to doubt — is that, 

 for the formation of such minerals and of the rocks con- 

 taining them, water and other volatile substances, held 

 within the soHd mass by intense pressure, is absolutely 

 indispensable. 



Now in the porphyritic constituents {Einsprenlinge or 

 phenocrysts) of many lavas, we find examples of minerals 

 which have been formed at great depths in the earth's 

 crust and then brought up to the surface and exposed to 

 totally different conditions, especially as regards pressure. 

 Very clearly do these phenocrysts tell the tale of their 

 origin, and of the influence exerted upon them by their 

 subsequent environments. 



Crystals of quartz and felspar, which have grown to 

 large proportions in the deeper portions of the earth's 

 crust, are found when brought up in lavas to the earth's 

 surface, and thus relieved from the action of pressure, to 

 be attacked by the magma in which they were originally 

 formed. The proof of this is seen in the corroded con- 

 dition of the crystals, the glassy matter surrounding them 

 having attacked their angles, their edges, and in a less 

 degree their whole surface, penetrating irregularly into ' 

 their interior, and reducing them sometimes to mere 

 skeletons. 



Crystals of hornblende and mica betray in an even 

 more striking manner the effects of a change of environ- 

 ment. When brought up from great depths in masses of 

 molten lava, crystals of these minerals are constantly 

 found to be surrounded by " resorption halos." The out- 

 side of the hornblende or mica crystals, where in contact 

 with the molten glass, is found to be attacked by it, and 

 crystals of pyroxene and magnetite have resulted from 

 the reaction. The action may in some cases continue 

 till the whole of the hornblende has been converted into 

 a pseudomorp^i. 



In some instances there may be reason to believe that 

 the phenocrysts have become enveloped in a magma of 

 different chemical composition to that in which they were 

 originally formed. But in many cases there is no room 

 for doubt that the minerals which were formed and main- 

 tained their stability under certain conditions of pressure, 

 lost that stability upon the diminution of pressure. 



That, conversely, the increase of pressure leads to the 

 production of a condition of instability in minerals formed 

 at or near the earth's surface there cannot be any doubt. 

 The study of the formation of crystalline schists from 

 various aqueous and igneous rocks supplies us with 

 numerous and very interesting illustrations of changes 

 of this kind : hornblendes, chlorites, micas, and talc 

 are produced under conditions of pressure in which 

 pyroxenes, epidotes, felspars, and olivines lose their 

 stability. 



III. In all those cases where solution is attended by 

 cotttraction, the solvent action of water and other liquids 

 is increased by pressure. 



That this is the case at elevated temperatures is proved 

 by the researches of Daubr^e to which we have already 

 referred. Pure water was made to attack various silicates 

 quite insoluble at ordinary temperatures and pressures. 

 Even if we admit with Bunsen that there are temperatures 

 at which this influence of pressure is no longer operative, 

 or at which the effects are wholly inappreciable, the 

 admission would not in any way affect the theoretical 

 views of the geologist, seeing that the increase of tem- 



NO. 1074, VOL. 42] 



perature within the earth's crust is so rapid, that even 

 at moderate depths the temperature at which solvent 

 action is increased by pressure must certainly exist. 



The effects of this solvent action under pressure are 

 everywhere manifest when we come to the study of the 

 rocks building up our earth's crust. At more or less 

 considerable depths, water containing carbon dioxide 

 has attacked the silicates composing the rock-forming 

 minerals ; so that it is impossible to find rocks which 

 have been deep-seated, at any period of their history,, 

 in which the minerals are in a perfectly unchanged 

 condition. 



Great masses composed originally of calcic carbonate,, 

 are found to have been changed into dolomite (the mag- 

 neso-calcic carbonate), or into chalybite (the ferrous 

 carbonate) ; while in other cases the whole mass of a bed 

 of calcic carbonate has been dissolved away, and silica 

 substituted as a " pseudomorph." 



We must proceed to study the details of such processes 

 especially as they are affected by pressure and by the 

 crystalline structure of the minerals affected. 



IV. Under great statical pressures^ the whole substance 

 of solid bodies tnay be per7neated by fluids, and chemical 

 reactions between them are thus greatly facilitated. 



It is not necessary to point out that the molecules of 

 the densest solids cannot be in actual contact ; this is 

 proved by the circumstance that such solids undergo con- 

 traction by lowering of temperature, and that gases may 

 be occluded in them. Physicists and mathematicians, 

 as recently pointed out to this Society by Prof. Riicker, 

 have even been able to arrive at positive conclusions con- 

 ' cerning, not only the actual order of magnitude of mole- 

 cules, but the distances that separate them from one 

 another in solids. 



The effect of pressure in causing the molecules of one 

 body to pass between those of another, has been expressed 

 by Van der Waals in the dictum, "AH bodies can mix 

 with one another, when the pressure exceeds a certain 

 value." A similar conclusion was expressed by the late 

 j Dr. Guthrie, as the result of his experiments on potassic 

 nitrate, when he asserted that " fused nitre and fused ice 

 are miscible in all proportions." 



Now, nothing is more certain, from petrographical 

 researches, than that the whole substance of the minerals 

 in the deep-seated rock-masses of the globe may be 

 permeated by fluids. This is shown by the condition of 

 the minerals forming these deep-seated masses. 



The felspars, in their normal condition, are colourless 

 and transparent minerals with a vitreous lustre, and this 

 is their character when they are found in lavas and in 

 blocks ejected from volcanoes. In granites and other 

 deep-seated rocks, however, these same felspars exhibit 

 grey, green, pink, or red tints, with more or less opacity, 

 and a remarkably pearly lustre. The cause of this change 

 of aspect is found in the fact that the unstable alkaline 

 silicates which enter into their composition have been 

 attacked by the fluids that have penetrated through the 

 whole substance of the crystal, leading to the formation 

 of the hydrated silicates of alumina, and, in some cases, 

 the peroxidation of any traces of iron compounds that 

 may have been present in them. 



Similar changes can be shown to have affected most, if 

 not all, the minerals which, at any period of their 

 history have formed portions of deep-seated rock- 

 masses. 



V. By the intimate intermixture, zmder great statical 

 pressures, of solids and fluids, the properties of the former 

 undergo great modifications. 



Bunsen, in common with all chemists who have studied 

 the great problem of geology, has insisted that fused 

 silicates, in spite of the high temperatures at which they 

 assume the fluid state, obey the same laws as those 

 governing ordinary solutions. Guthrie has shown that 

 the principles which determine the formation of " cryo- 



