1859.] BJTST CHE3IICAL GEOLOGY. 495 



potash. Hence in more recent crystalline rocks we find a less ex- 

 tensive development of soda-felspars, while orthoclase and mica, chlo- 

 rite and ejDidote, and silicates of alumina, like chiastolite, kyanite, 

 and staurotide, which contain but little or no alkali, and are rare in 

 the older rocks, become abundant*. 



The decomposition of the rocks is more slow now than formerly, 

 because soda- silicates are less abundant, and because the proportion 

 of carbonic acid in the air (an efficient agent in these changes) has 

 been diminished by the formation of limestones and coal. It will 

 be evident that the principles above laid down are only applicable 

 to the study of rocks in great masses, and refer to the predominance 

 of certain mineral species at certain geologic epochs, since local and 

 exceptional causes may reproduce in different epochs the conditions 

 which belong to other periods. 



§ YII. Mr. Babbage f has shown that the horizons or surfaces of 

 equal temperature in the earth's crust must rise and fall, as a con- 

 sequence of the accumulation of sediment in some parts and its 

 removal from others, producing thereby expansion and contraction 

 in the materials of the crust, and thus giving rise to gradual and 

 wide- spread vertical movements. Sir John Herschel J: subsequently 

 showed that, as a result of the internal heat thus retained by accu- 

 mulated strata, sediments deeply enough buried will become cry- 

 stallized and ultimately raised with their included water to the 

 melting point. From the chemical reactions at this elevated tem- 

 peratui'e, gases and vapours will be evolved, and earthquakes and 

 volcanic eruptions will result. At the same time the disturbance 

 of the equilibrium of pressure consequent upon the transfer of sedi- 

 ment, while the yielding surface reposes upon a mass of matter 

 partly liquid and partly solid, will enable us to explain the phe- 

 nomena of elevation and subsidence. 



According, then, to Sir J. Herschel's view, all volcanic phenomena 

 have their source in sedimentary deposits : and this ingenious hypo- 

 thesis, which is a necessary consequence of a high central tem- 

 perature, explains in a most satisfactory manner the dynamical 

 phenomena of volcanos, and many other obscure points in their 

 history, as, for instance, the independent action of adjacent volcanic 

 vents, and the varying nature of their ejected products. IS'ot 

 only are the lavas of different volcanos very unlike, but those of the 

 same crater vary at different times ; the same is true of the gaseous 

 matters, hj'drochloric, hydrosulphuric, and carbonic acids. As the 

 ascending heat penetrates saliferous strata, we shall have hydro- 

 chloric acid, from the decomposition of sea-salt by silica in the pre- 

 sence of water ; while gypsum and other sulphates, by a similar 

 reaction, would lose theii' sulphur in the form of sulphuric acid and 

 oxygen. The intervention of organic matters, either by direct con- 

 tact, or by giving rise to reducing gases, would convert the sulphates 



* Corundum and diaspore are probably derived from basic aluminous sili- 

 cates like schroterite and eollyrite, themselves the kaoHn of basic felspars, 

 t " On the Temple of Serapis," Proc. Geol. Soc. vol. ii. p. 73. 

 I Ibid. vol. ii. pp. 548 & 596. 



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