in viscid globules filled with air, which burst, and then collapsed, and became solidified in their present form. In silicified wood, the permeation of the vegetable tissues by the mineral matter appears to have been effected by solutions of silex of a high temperature. In some examples mineralization has been effected simply by replacement: the original substance has been removed, atom by atom, and the silex substituted in its place. One of the most eminent naturalists and mineralogists of the United States, Mr. J. D. Dana,[141] suggests that the reason why silica is so common a material in the constitution of fossil wood and shells, as well as in pseudo-morphic crystals,[142] consists in the ready solution of silex in water at a high temperature (a fact affirmed by Bergman[143]) under great pressure, whenever an alkali is present, as is seen at the present time in many volcanic regions, and its ready deposition again when the water cools. A mere heated aqueous solution of silica, under a high pressure, is sufficient to explain the phenomenon of the silicification of organic structures. Mr. Dana states that "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 temperature, and takes the place of the lime, atom by atom, as soon as set free. Every silicified fossil is an example of this pseudo-morphism; but there seems to be no union of the silica with the lime, for silicate of lime is of rare occurrence."[144] [Footnote 141: American Journal of Science, for January, 1845.] [Footnote 142: Pseudo-morphic crystals are crystals moulded in the cavities left by other crystals, which they have replaced. See Dr. Blum on pseudo-morphous minerals; and Mr. Jefferey's experiments on the solution of silica in heated vapour; Wonders of Geology, p. 100.] [Footnote 143: Bergman first determined the solubility of silex in simple water, aided by heat, and demonstrated its existence in the Geysers, and other boiling springs of Iceland. _Parkinson_, _Org. Rem._, vol. i. p. 324.] [Footnote 144: See my "Notes on a microscopical examination of chalk and flint," Annals of Natural History, August 1845.]