^{page 433 note 2} Geol. Mag., Vol. IV. pp. 357–369. See also pp. 432. and 477.—Edit.Google Scholar

^{page 434 note 1} In the belief that fair discussion advances science, by developing energetically both sides of the question; such discussions should however only be indulged in by those who can give and take with equal good grace, without losing temper or deviating from the subject at issue by indulging in recriminations or personalities, such as Dr. Hunt at the close of his lecture alludes to, as having disturbed the social relations of the geologists of the last century, and which unfortunately sometimes creep into discussion even in this enlightened age.

^{page 434 note 2} Geol. Mag., Vol. IV. p. 287. The author still agrees with “most geologists” in opposition to the opinions referred to.Google Scholar

^{page 435 note 1} If we suppose the mean density of the earth to be 5·3, and that of the surface crust to be 2·65, and further imagine the earth to be composed of three consecutive layers of equal thickness, and of density increasing in arithmetical progression, we should have 2·65 for the density of the outermost zone, nearly 10·7 for that of the middle one, and about 18·8 for the centre.

^{page 436 note 1} Brit. Association Report, 1854, p. 57.Google Scholar

^{page 436 note 2} Poggendorf. Ann., vol. 81, p. 562. 1850.Google Scholar

^{page 436 note 3} The nature of such organic substances being totally different from any to be met with under the circumstances here under consideration, much dependence could not be placed upon the similarity of behaviour of inorganic compounds.

^{page 437 note 1} And the author thinks it probable that the same would also be the case with the relations of fusing points to pressure.

^{page 437 note 2} In opposition to this view, it might be said that the densest of all metals, Platinum, is also one of the most infusible. To this we answer, that many of the compounds of Platinum, say with zinc, tin, arsenic, etc., are so extremely fusible as to melt in the flame of a candle.

^{page 438 note 1} The silica produced from such decomposition of silicates is of the specifie gravity of 2·2, is soluble in alkaline solutions, and does not polarize light, which is not the case with the silica contained in any of the older rocks, which Dr. Hunt supposes to have been so formed. The chemical and physical properties of the silica of such rocks indicate them to have been derived from, the breaking down of acid rocks analogous to granite.

^{page 439 note 1} The zone of carbonic acid gas would be heavier than that of steam; 1 cubic foot of the latter weighing at 212° only 265·17 grains, whilst 1 cubic foot of carbonic gas would weigh 642·09 grains, at same temperature.

^{page 439 note 2} A rough calculation shows that the layer of sea salt alone would be sufficient to clothe the entire sphere with a crust of salt some 10 feet in thickness.

^{page 439 note 3} This action would, no doubt, be much facilitated in the older geological epochs, by the amount of carbonic acid in the atmosphere being so much greater than at present.

^{page 439 note 4} The contraction, consequent upon the cooling of the original sphere, would, doubtless, greatly disturb the previously comparatively even surface of the crust, and produce cracks and fissures, the sides of which, from their unequal subsidence or elevation, would often be dislocated and form lines of faults.

^{page 440 note 1} It may here be remarked that Dr. Hunt, in his lecture, does not allude to what became of the sulphuric acid, which would be the ultimate product of “all the sulphur” burnt into sulphurous acid, and afterwards condensed from the atmosphere into the ocean; for since it may safely be asserted that there is fully as much (if not more) sulphur as chlorine, the sea formed, according to Dr. Hunt's hypothesis, would be as much a solution of sulphate of soda as of seasalt, and he can hardly suppose it to have been precipitated, for it is well known that no beds of sulphate of any importance whatsoever occur in the very oldest formations.

^{page 441 note 1} Even the chalk is entirely so composed, notwithstanding that its external appearance is so like that of a precipitated carbonate of lime.

^{page 441 note 2} Dr.Hunt, seems to be quite unaware that in the Brit. Assoc. Report, 1856, p. 77, Sorby has fully explained these reactions, that Harkness (Brit. Assoc. Report, 1857, p. 68) applied similar experimental investigations of Regnault, to explaining the dolomitization of the Carboniferous limestones near Cork; and, lastly, that the results of his researches on the artificial atmosphere of carbonic acid, which he has thought worthy of bringing before the French Academy (Compt. Rend., 1867, p. 815), so far from being new, have for the last twenty-two years at least, if not much longer, been employed on a large scale in the manufacture of magnesian compounds in both England and Ireland.Google Scholar

^{page 442 note 1} If we, with Dr. Hunt, believe that the temperature increases in proportion to the pressure, then, as Sorby has shown that the quartz of the granite of Aberdeen has solidified at a pressure equal to a column of seventy-eight thousand feet of rock, this alone would be quite sufficient to refute the statement of comparatively low temperatures.

^{page 443 note 1} An argument has been brought forward against the igneous origin of granite, from the fact that the specific gravity of the quartz in granite is 2é6, whilst the density of silica artificially fused before the oxyhydrogen blowpipe is only 2é2. 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 2é2, 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 tbat 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 03, 1865, contained fine crystals of Stilbite (with 16 per cent, of water). Bunsbn's researches (Taylor's Scient. Memoirs, 11., 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 extracted 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 conclusions arrived at in that gentleman's memoir.Google Scholar

^{page 444 note 1} Apparently an application of the “sensation” principle to geology.

^{page 444 note 2} It is nearly half a century ago since Scrope not only pointed out the important part played by water in volcanic action, but further expatiated upon the difference between volcanic fusion and ordinary melting.