I think these nodules bear evident marks of their having been in perfect fusion by either heat alone, or by water and heat, under great pressure, according to the ingenious theory of Dr. Hutton; but I do not imagine, that they were injected into cavities from materials from without, but that some vegetables or parts of vegetables containing more iron or manganese than others, facilitated the compleat fusion, thus destroying the vestiges of vegetable organization, which were conspicuous in the siliceous trees above mentioned. Some of these nodules being hollow and lined with crystals, and others containing a nucleus of white siliceous matter of a looser texture, shew they were composed of the materials then existing in the cavity; which consisting before of loose sand, must take up less space when fused into a solid mass.
These siliceous nodules resemble the nodules of iron-stone mentioned in note on Canto II. l. 183, in respect to their possessing a great number of concentric spheres coloured generally with iron, but they differ in this circumstance, that the concentric spheres generally obey the form of the external crust, and in their not possessing a chalybeate nucleus. The stalactites formed on the roofs of caverns are often coloured in concentric strata, by their coats being spread over each other at different times; and some of them, as the cupreous ones, possess great beauty from this formation; but as these are necessarily more or less of a cylindrical or conic form, the nodules or globular flints above described cannot have been constructed in this manner. To what law of nature then is to be referred the production of such numerous concentric spheres? I suspect to the law of congelation.
When salt and water are exposed to severe frosty air, the salt is said to be precipitated as the water freezes; that is, as the heat, in which it was dissolved, is withdrawn; where the experiment is tried in a bowl or bason, this may be true, as the surface freezes first, and the salt is found at the bottom. But in a fluid exposed in a thin phial, I found by experiment, that the extraneous matter previously dissolved by the heat in the mixture was not simply set at liberty to subside, but was detruded or pushed backward as the ice was produced. The experiment was this: about two ounces of a solution of blue vitriol were accidentally frozen in a thin phial, the glass was cracked and fallen to pieces, the ice was dissolved, and I found a pillar of blue vitriol standing erect on the bottom of the broken bottle. Nor is this power of congelation more extraordinary, than that by its powerful and sudden expansion it should burst iron shells and coehorns, or throw out the plugs with which the water was secured in them above one hundred and thirty yards, according to the experiments at Quebec by Major Williams. Edinb. Transact. Vol. II. p. 23.
In some siliceous nodules which now lie before me, the external crust for about the tenth of an inch consists of white agate, in others it is much thinner, and in some much thicker; corresponding with this crust there are from twenty to thirty superincumbent strata, of alternately darker and lighter colour; whence it appears, that the external crust as it cooled or froze, propelled from it the iron or manganese which was dissolved in it; this receded till it had formed an arch or vault strong enough to resist its further protrusion; then the next inner sphere or stratum as it cooled or froze, propelled forwards its colouring matter in the same manner, till another arch or sphere produced sufficient resistance to this frigoriscent expulsion. Some of them have detruded their colouring matter quite to the centre, the rings continuing to become darker as they are nearer it; in others the chalybeate arch seems to have stopped half an inch from the centre, and become thicker by having attracted to itself the irony matter from the white nucleus, owing probably to its cooling less precipitately in the central parts than at the surface of the pebble.
A similar detrusion of a marly matter in circular arches or vaults obtains in the salt mines in Cheshire; from whence Dr. Hutton very ingeniously concludes, that the salt must have been liquified by heat; which would seem to be much confirmed by the above theory. Edinb. Transact. Vol. I. p. 244.
I cannot conclude this account of Scots-pebbles without observing that some of them on being sawed longitudinally asunder, seem still to possess some vestiges of the cylindrical organization of vegetables; others possess a nucleus of white agate much resembling some bulbous roots with their concentric coats, or the knots in elm-roots or crab- trees; some of these I suppose were formed in the manner above explained, during the congelation of masses of melted flint and iron; others may have been formed from a vegetable nucleus, and retain some vestiges of the organization of the plant.
4. SAND OF THE SEA.
The great abundance of siliceous sand at the bottom of the ocean may in part be washed down from the siliceous rocks above described, but in general I suppose it derives its acid only from the vegetable and animal matter of morasses, which is carried down by floods or by the atmosphere, and becomes united in the sea with its calcareous base from shells and coralloids, and thus assumes its crystalline form at the bottom of the ocean, and is there intermixed with gravel or other matters washed from the mountains in its vicinity.
5. CHERT, OR PETROSILEX.
The rocks of marble are often alternately intermixed with strata of chert, or coarse flint, and this in beds from one to three feet thick, as at Ham and Matlock, or of less than the tenth of an inch in thickness, as a mile or two from Bakewell in the road to Buxton. It is difficult to conceive in what manner ten or twenty strata of either limestone or flint, of different shades of white and black, could be laid quite regularly over each other from sediments or precipitations from the sea; it appears to me much easier to comprehend, by supposing with Dr. Hutton, that both the solid rocks of marble and the flint had been fused by great heat, (or by heat and water,) under immense pressure; by its cooling or congealing the colouring matter might be detruded, and form parallel or curvilinean strata, as above explained.