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 silica ; and 
in the latter case, the heated waters, if marine, will mclude both 
soda and magnesian salts. Mr. Dana goes on to show that a 
mere heated solution of silica i water, under great pressure, 1s 
sufficient to expla the phenomenon 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 erystal of cale-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 liberated lime, since silicate of lime 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 1s 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 cayities 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 subaérial volcanoes ; and when further we have before 
our eyes the wide-spread effects of voleanic 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 mincral kingdom ; especially if we take 
into view the condition of a vast submarine volcanic region in 
