Chalk and Flint of the South-east of England. 81 



the present time in many volcanic regions,) and its ready depo- 

 sition again when the waters cool. A solution of silica, whether 

 resulting from the deposition of felspar at the ordinary tempe- 

 rature, or whether proceeding from submarine volcanic action, 

 will in either case contain other substances. The alkali of the 

 felspar, potash, or soda, passes off with the liberated sihca ; and 

 in the latter case, the heated waters, if marine, will include both 

 soda and magnesian salts. Mr. Dana goes on to show that a 

 mere heated solution of silica in water, under great pressure, is 

 sufficient to explain the phsenomenon of silicification of organic 

 structures. Thus in the strata of white chalk, in which the shells 

 of mollusca are not silicified, but remain calcareous, the streams 

 of water holding silex in solution, were probably of a lower tem- 

 perature than in the case of the Devonshire silicified shells, the 

 pseudomorphism of which may have been effected by a very hot 

 solution of silica. " For a crystal of calc-spar in such a fluid, 

 being exposed to solution from the action of the heated water 

 alone, the silica deposits itself gradually on a reduction of tempe- 

 rature, and takes the place of the lime, atom by atom, as soon as 

 set free. Every silicified fossil is an example of this pseudomor- 

 phous process ; but there seems to be no union of the silica with 

 the Uberated lime, since silicate of hme occurs extremely seldom, 

 if at all, either in the fossils themselves or in the surrounding 

 rock. There appears to be something in the chemical or electro- 

 chemical forces excited among the molecules by the process of 

 solution, which leads the molecules of any body that may be 

 passing at the time from a liquid state to take the place succes- 

 sively of each molecule that is removed ; and thus it is that the 

 form of the original structure, to the minutest character, is so ex- 

 actly assumed by the substituting mineral. Fluor spar, and even 

 heavy spar or barytes, although stated to be insoluble, have evi- 

 dently undergone solution in heated waters, and thus been depo- 

 sited in cavities and veins of sedimentary limestones that show no 

 trace of the effects of a higher temperature ; for they are not fused, 

 nor even rendered crystalline. The agency of hot waters and 

 vapours in producing changes in rocks and in organic remains 

 has perhaps scarcely received sufficient attention. When we con- 

 sider the number of hot springs on the surface of the earth in 

 regions of modern volcanic action, as well as in others not of this 

 nature; when we remember the many eruptions of hot water 

 even from subaerial volcanoes ; and when further we have before 

 our eyes the wide- spread effects of volcanic action beneath the 

 sea, — can we refuse to the agency of heat thus conveyed by vapours 

 - and flowing mineral waters, a large share of the various meta- 

 morphic changes in the mineral kingdom ; especially if we take 

 into view the condition of a vast submarine volcanic region in 



