ON THE EFFECTS OF LONG-CONTINUED HEAT. 191 



veins, this, I believe, is the opinion of most observers. But we see the same 

 metamorphic effects which are produced by them, equally produced by the 

 presence of any eruptive rock. If a stratum of limestone be invaded, and a 

 portion of it included in the invading substance, that portion is not unfre- 

 quently impregnated with magnesia and converted into dolomite, equally by 

 a mineral vein or a granitic rock. 



The advantages which this theory possesses over any that have yet pre- 

 sented themselves, are that it accounts for all the following phenomena: — 



1. The characteristic structures of granite, and of gneiss and mica-slate, — 

 which may be compared to the deposits of graphite in gas-retorts, solid 

 where the carburetted gas aggregates its decomposed molecules of carbon in 

 confinement, hut foliated and quasi-stratified, where the gas chances to escape 

 through cracks in the retort into the more open chamber of brick-work ; — 



2. The perfect uniformity of crystalline texture in granite, whether deep or 

 superficial, in thin veins or solid masses, showing that neither great pressure 

 nor slow cooling have been essential conditions of its crystallization ; — 



3. The wide diffusion of zones or atmospheres round the eruptive, and 

 especially the granitic rocks, of mineral substances, and metamorphic effects, 

 a phenomenon which, together with that of the filling up of mineral veins 

 from below, is not accounted for hy any other theory ; — 



4. The metalliferous and quai'tziferous impregnations of the sedimentary 

 strata. 



If, with Cordier, we divide the eruptive rocks into the quartzose (which 

 correspond to the granites and earliest porphyries) ; and the unquartzose, 

 comprehending the felspathic (which correspond to the later porphyries and 

 trachytes); with the pyroxenic (which correspond to the basalts and lavas); 

 and if we consider all these as originating from gases, accompanied by 

 aqueous vapour, — then the phenomena show the amount of such vapour 

 present in the quartzose formations to have been almost infinitesimal, 

 whilst that which attended some parts of the pyroxenic formations was con- 

 siderable. As regards the sedimentary siliciferous rocks, they show, in the 

 semiopaline, semiquartzose composition of the siliceous beds, the action of 

 anhydrous gas, aided by aqueous vapour. Aqueous vapour acts on silicates 

 only at a heat approaching redness, and conveys no silica. Chloride of silicon 

 would carry silica, and would diffuse it at a much lower heat, since it boils 

 at a temperature below 140° F. 



Connected with the preceding speculations the following remarks may 

 deserve attention. There is a singular resemblance of mineral and crystal- 

 line constitution between the pyroxenic rocks and meteoric stones, — a re- 

 semblance, in fact, so close as to indicate a similar mode of production out 

 of the same materials. The late optico-chemical discoveries of Bunsen and 

 Kirchhoff have shown, with a great degree of probability, that molecules of 

 iron, nickel, and magnesium abound in the solar atmosphere ; should the 

 progress of those discoveries add silicon to this list, we have here again the 

 chief materials, both of metcorolites and of pyroxenic rocks. In any case, 

 whether we suppose the meteorite to have been contemporaneous with the 

 earth, or to be ejected from the moon, or emitted from the sun, our thoughts 

 are led back to a time when the whole solar system consisted of the same 

 ultimate atoms, and are confirmed in the opinion that the meteorites and the 

 fundamental rocks of the earth have undergone similar processes of mole- 

 cular and crystalline combination, the vitreous coat of the meteorite, and the 

 vitreous character of the later lavas, being due also to the same causes : — 

 1st, to the fusibility of the material ; 2ndly, to a more intense heat generated 

 by a nearer proximity to an oxidating atmosphere ; .Srdly, to a more rapid 

 rate of cooling. 



