Dr, T, Sterry Hunt — On Chemical Geology, 57 



not above dull redness, under great pressure, and in the presence of water, is pre- 

 cisely what I have always maintained. When it is remembered that copper, gold, 

 and silver require for fusion from 1000° to 1400", and quartz a temperature of 

 2800°, it may not be thought incorrect for me to designate 360'', the highest 

 assigned by Mr. Sorby for the crystaUization of quartz, as a "comparatively low 

 temperature," to which expression, however, Mr. Forbes takes exception. Mr. 

 Sorby further concludes from his investigations of crystalline metamorphic schists, 

 that they must have crystallized at about the same temperature as the granites, 

 affording, in his words, "a strong argument in favour of the supposition that the 

 temperature concerned in the normal metamorphism of gneissoid rocks, was due to 

 their having been at a sufficiently great depth under superincumbent strata." 



The reader may now judge how far the views of Mr. Sorby, whom Mr. Forbes 

 invokes, differ from my own on the subject of metamorphic rocks, of which I say 

 in my lecture, as quoted by Mr. Forbes, that they have been formed from ordinary 

 sedimentary strata, " depressed so that they come within the action of the earth's 

 central heat," a proposition which our critic thinks "may be disputed." What 

 theory he substitutes, he does not deign to inform us, but proceeds to ask how I 

 explain the depression of strata on the surface of a globe with a solid centre. 

 Both in my lecture and in the papers already cited, I have taken pains to explain 

 that the deeply buried layers of sediment, together with the superficial and water- 

 impregnated portions of the solid nucleus, constitute a softened or plastic zone, 

 from which all plutonic and volcanic rocks proceed, and which allows of the move- 

 ments observed in the solid crust. Is Mr. Forbes aware that geology affords 

 many examples of depression of the earth's surface over great areas, permitting 

 accumulations of sediments, to the extent of 40,000 feet or more, followed by ele- 

 vation of the yielding crust and denudation to as great an amount ? 



I here take occasion to call attention to an important consideration in connection 

 with this, deducible from Mr. Sorby's admirable Bakerian lecture before the Royal 

 Society, in 1863, on the Direct Correlation of Mechanical and Chemical Forces, 

 in which he shows how chemical action is produced by mechanical force. Stating 

 from a consideration of the results of Bunsen and Hopkins that those bodies which 

 expand when fused have their point of fusion raised by mechanical pressure, and 

 from the discovery of Sir Wm. Thompson tl^at water, which contracts in melting, 

 has, on the contrary, its melting point lowered by pi-essure, we may say that as the 

 solution of a solid in a liquid is a kind of fusion, the same general law will hold 

 good, and that, for all those salts which contract in dissolving (to which rule there 

 are very few exceptions), the solubility should be increased by pressure. This was 

 abundantly established by the experiments of Mr. Sorby. If, now, we suppose 

 that the mineral compounds of the crystalline rocks, like most salts, occupy a less 

 bulk when dissolved that when in the solid state, we can understand the greatly in- 

 creased solvent power of the water present in sediments submitted to a pressure 

 equal to many thousand feet of rock. Moreover, as suggested to me by Sir Wm, 

 Logan, the diminution of solvent power of the liquid as the pressure is removed, 

 will help to explain the deposition of mineral matters from watery solutions, which 

 in their upward flow through fissures in the earth's crust have given rise to mineral 

 veins. 



But Mr. Forbes, after considering the conclusion of Sorby that water has played 

 an essential part in the crystallization and softening of rocks, which have been 

 effected at temperatures not above low redness, charges me with "sensation" 

 writing in asserting that the plutonists claimed that igneous rocks were formed 

 "entirely by fire," and accuses me of injustice to the memories of Hutton, Play- 

 fair, Hall, Humboldt, and Von Buch, whose writings "show that they never 

 overlooked the all-important influence of water." Now I mentioned none of these 

 geologists in my lecture. As to Hutton, to whom belongs, I believe, the idea of 

 the metamorphic origin of the crystalline schists, I have elsewhere written (Dublin 

 Quart. Joiitn., July, 1 863), " I accept in the widest sense the view of Hutton and 

 Boue that all the crystalline stratified rocks have been produced by the alteration 

 of mechanical and chemical sediments." The question before us is, however, 

 neither the views of Hutton nor yet the origin of metamorphic rocks, but what 

 both he and modern plutonists hold with regard to the origin of granite, whose 

 derivation from metamorphosed sediments neither he nor they admit. With regard 

 to this point Mr. Forbes elegantly says, " the idea of dry fusion could only have 



