310 ANNUAL OF SCIENTIFIC DISCOVERY. 



recent crystalline rocks, we find a less extensive development of soda-feld- 

 spars ; while orthoclase, and mica, chlorite, and epidote, 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. 



VII. Mr. Babbage* has shown that the horizons, or surfaces, of equal 

 temperature in the earth's crust, must rise and fall, as a consequence of the 

 accumulation of sediment in some parts, and its removal from others, pro- 

 ducing, thereby, expansion and contraction in the materials of the crust, 

 and thus giving rise to gradual and wide-spread vertical movements. Sir 

 John Herschelt subsequently showed that, as a result of the internal heat 

 thus retained by accumulated strata, sediments deeply enough buried will 

 become crystallized, and ultimately raised,with their included water, to the 

 melting-point. From the chemical reactions at this elevated temperature, 

 gases and vapors will be evolved, and earthquakes and' volcanic eruptions 

 will result. At the same time, the disturbance of the equilibrium of pres- 

 sure, consequent upon the transfer of sediments, while the yielding surface 

 reposes upon a mass of matter partly liquid and partly solid, will enable us 

 to explain the phenomena of elevation and subsidence. 



According then to Sir J. Herschel's vieAv, all volcanic phenomena have their 

 source in sedimentary deposits ; and this ingenious hypothesis, which is a 

 necessary consequence of high central temperature, explains, in a most sat- 

 isfactory manner, the dynamical phenomena of volcanoes, 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. 

 Not only are the lavas of different volcanoes very unlike, but those of the 

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

 hydrochloric, hydrosulphuric, and carbonic acids. As the ascending heat 

 penetrates saliferous strata, we shall have hydrochloric acid, from the decom- 

 positon of sea-salt by silica, in the presence of water; while gypsum, and 

 other sulphates, by a similar reaction, would lose their sulphur in the form 

 of sulphurous acid and oxygen. The intervention of organic matters, either 

 by direct contact, or by giving rise to reducing gases, would convert the sul- 

 phates into sulphurcts, which would yield sulphuretted hydrogen when de- 

 composed by water and silica, or carbonic acid the latter being the result of 

 the action of silica upon earthy carbonates. We conceive the ammonia so 

 often found among the products of volcanoes, to be evolved from the heated 

 strata, where it exists in part as ready-formed ammonia (which is absorbed 

 from air and water, and pertinaciously retained by argillaceous sediments), 

 and is in part formed by the action of heat upon azotized organic matter 

 present in these strata, as already maintained by Bischof. Xor can we hesi- 

 tate to accept this author's theory of the formation of boracic acid from 

 the decomposition of borates by heat and aqueous vapor. 



* " On the Temple of Serapis," Tree. Geol. Soc. vol. ii. p. 73- 

 t Ibid. vol. ii., pp. 548, 593. 



