[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.
[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.
[144] See my "Notes on a microscopical examination of chalk and flint," Annals of Natural History, August 1845.
[XX.] Stigmaria, Sigillaria, &c.—The most remarkable peculiarity of the flora of the carboniferous period is the immense numerical preponderance of the vascular or higher tribes of cryptogamic plants, which amount to two-thirds of the species described. With these are associated a few Palms, Coniferæ, Cycadeæ, and dicotyledonous plants, allied to the Cacteæ and Euphorbiaceæ. The number and magnitude of the vegetables bearing an analogy to the Ductulosæ, but differing from existing species and genera, constitute therefore the most important botanical feature of the carboniferous flora. Thus we have plants allied to the Equisetum, or mare's-tail (Calamites), eighteen inches in circumference, and from thirty to forty feet high; Zamia-like coniferæ (Sigillariæ) fifty feet high; and arborescent club-mosses (Lepidodendra) attaining an altitude of sixty or seventy feet. Of this ancient flora, the fossil plants whose stems have been named Sigillaria (see [Plate XXI.]), and their roots Stigmaria, are especially remarkable in consequence of the peculiar circumstances under which upright examples of these trees are occasionally met with. Referring for details to "Wonders of Geology," Lecture VII., I purpose describing in this place the facts recently brought to light, which prove that certain species of Stigmaria are the roots of Sigillariæ; while others in all probability belong to Lepidodendra:—an opinion maintained more than thirty years ago by the Rev. H. Steinhauer.[145] To the late Mr. Binney we are indebted for the first confirmation of the inference of my friend, M. Adolphe Brongniart, (derived from an examination of the structure of those bodies,) that the Stigmariæ are the veritable roots of Sigillariæ. At St. Helen's, near Liverpool, Mr. Binney discovered, in 1844, an upright trunk of a Sigillaria, nine feet high, to which were attached ten roots, several feet long, that extended into the under clay in their natural position;[146] and these roots were unquestionable Stigmariæ, the tubercles with their attached rootlets being clearly displayed. In the floor of the Victoria Mine at Dunkinfold, near Manchester, at the depth of 1,100 feet from the surface, Mr. Binney also discovered a magnificent specimen of Sigillaria, which exhibited on its stem the respective characters of three supposed species, and had stigmaria-roots extending twenty feet.
[145] Transactions of the American Philosophical Society, New Series, vol. i.
[146] See Medals of Creation, vol. i. p. 143.
In the Sydney coal-field at Cape Breton, Mr. Richard Brown has observed several upright stems of Sigillariæ and Lepidodendra, with stigmaria-roots attached; and the same fact has been noticed in the Picton coal, in Nova Scotia. The following figures and notes from Mr. Brown's description of these interesting phenomena, will place the subject before the reader in a clear point of view.[147]
[147] "Description of an upright Lepidodendron with Stigmaria-roots in the roof of the Sydney Main Coal, in the Island of Cape Breton. By Richard Brown, Esq."—Geological Journal, No. 13, for June, 1847, p. 46.
The main bed of coal is six feet in thickness, and is overlaid, as usual, by a roof of shale abounding in foliage and fragments of branches. As the coal is dug out, large masses of the shale fall in, and occasionally hollow spaces, called by the workmen pot-holes, are left in the roof; the fallen masses being the roots and truncated stems of Sigillariæ and other trees, which separate at the parting formed by the coaly bark covering the roots, when the supporting coal is removed.
The following sketch represents one of the specimens of the base of a stem of a Lepidodendron, with the roots (stigmariæ) attached. This figure (1) shows the position of the tree above the bed of main coal, with the inclination and length of two of the principal roots, so far as they could be distinctly traced; and the following sketch (2) represents the trunk, with its branching roots, constructed from careful measurements of the dimensions and position of each root, drawn on the spot. The stem at the part marked A, was encrusted with a coaly bark, covered by the usual cicatrices of the Lepidodendra, and the roots at B, C, D, with a similar carbonaceous investment, impressed with the characteristic pits or areolæ of Stigmariæ.