422 On Some Points in Chemical Geology, 



we have no reason to suppose that they mark the commencement 

 of sedimentary deposits ; they were doubtlessly derived from the 

 ruins of other rocks in which the proportion of soda was still 

 greater ; and the detritus of these Laurentian felspars, making up 

 our paloe ozoic strata, is now the source of alkaline waters by which the 

 soda of the silicates, rendered soluble, is carried down to the sea 

 in the form of carbonate to be transformed into chlorid of sodium. 

 The lime of the feldspars being at the same time removed as car- 

 bonate, these sedimentary strata in the course of ages become less 

 basic, poorer in soda and lime, and comparatively richer in alumina, 

 silica, and potash. Hence in more recent crystalline rocks we find 

 a less extensive development of soda-feldspars, 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 predomi- 

 nance 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 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 1 1 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- 

 perature, 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- 

 ments, 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. 



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

 || Ibid. vol. ii, pp. 548 & 596. 



