Forbes — Chemistry of the Primeval Earth. 443 



Geol. Soc. of London, vol. xiv., sliows how perfectly identical in 

 structure this volcanic quartz is with the quartz of granites (both 

 containing in common, fluid, vapour, gas, and stone cavities , and 

 that this accurate observer has concluded that the modern volcanic 

 trachytes and old granites have one common igneous origin, in 

 which, as is the case in volcanoes, water has played some part/ 



That the metamorphic rocks have been formed from ordinary 

 sedimentary strata, by their having been " depressed so that they 

 come withia the action of tlie earth's central heat" may be disputed ; 

 but before doing so it might be as well to learn from the author of 

 this ingenious theory by what mechanical arrangement he supposes 

 strata on the surface of the earth to be lowered down into a globe 

 solid to the core. The further development of this theory, assuming 

 a similar action to have produced the eruptive rocks emitted by the 

 volcanoes of the present day, is at once strongly protested against ; 

 for how, may it be asked, are we, according to this theory, to 

 account for the fact, that volcanic rocks taken from any quarter of 

 the world, no matter how far distant from one another — from 

 Iceland or Terra del Fuego, from the Islands of the West Indies or 

 from those of Polynesia — that in all cases such rocks possess an 

 absolute identity in chemical and mineralogical composition ; in 

 physical and in optical properties. Can any geologist be expected 



1 An argument has been brougbt forward against the igneous origin of granite, 

 from the fact that the specific gravity of the quartz in granite is 2 "6, whilst the 

 density of silica artificially fused before the oxyhydrogen blowpipe is only 2-2. If 

 this style of argument is admitted in philosophical reasoning, then the silica of the 

 carapaces of infusoria ought also to have been formed by fusion, since its specific 

 gravity is only 2-2, as is also the silica deposited from its gaseous compounds with 

 fluorine, etc. Sorby's before -mentioned researches have shown that the quartz in 

 granite has solidified under enormous pressure. It might therefore reasonably be 

 expected to possess a higher density than such as has been fused artificially, •without 

 having been subjected to pressure at all. Another argument is found in the fact that 

 some of the more fusible minerals in granite have often solidified and crystallized 

 before less fusible ones ; in reply, it may be stated that this is also the case in modern 

 lavas ; in those of Vesuvius, it is common to find that the refractory Leucite has 

 crystallized before the easily fusible Augite, and to be superposed on crystals of this 

 latter mineral. It has further been argued, that rocks like granite occasionally 

 enclosing minerals containing water, could not have been formed by igneous fusion ; 

 independently of Sorby's discovery that the quartz of volcanic rocks and the felspar, 

 nepheline, idocrase, etc., ejected from Vesuvius, do contain water : specimens taken 

 out of the lava current from Etna, whilst still flowing in March, 1865, contained fine 

 crystals of Stilbite (with 16 per cent, of water). Bunsen's researches (Taylor's Scient. 

 Memoirs, Nov., 1852) have long ago experimentally proved that hydrated silicates, 

 analogous to those occurring in eruptive rocks, might be formed at high temperatures 

 and retain their water at such temperatures as long as enclosed in the matrix ; if ex- 

 tracted from this, however, the water could be expelled by the application of a very 

 gentle heat. Laurent has also showed that borate of Potash, fused at temperatures 

 above the melting point of Silver, retained water which, singularly enough, might be 

 expelled in bubbles by reheating the vitrified mass over a spirit lamp so as hardly to 

 soften it. — "Whilst correcting the proofs of this paper for the press, the author has had 

 his attention directed to a communication made by Professor Ansted to the British 

 Association, " On the Passage of Schists into Granite in the Island of Corsica," in 

 which (if the report in the "Dundee Advertiser," Sept. 10, be correct) the learned 

 Professor cites, in support of his views, the results of Mr. Sorby's researches in a 

 manner apparently quite at variance with the couclusions arrived at in that gentle- 

 man's memoir. 



