1. SILICEOUS ROCKS.
The great masses of siliceous sand which lie in rocks upon the beds of limestone, or which are stratified with clay, coal, and iron-ore, are evidently produced in the decomposition of vegetable or animal matters, as explained in the note on morasses. Hence the impressions of vegetable roots and even whole trees are often found in sand-stone, as well as in coals and iron-ore. In these sand-rocks both the siliceous acid and the calcareous base seem to be produced from the materials of the morass; for though the presence of a siliceous acid and of a calcareous base have not yet been separately exhibited from flints, yet from the analogy of flint to fluor, and gypsum, and marble, and from the conversion of the latter into flint, there can be little doubt of their existence.
These siliceous sand-rocks are either held together by a siliceous cement, or have a greater or less portion of clay in them, which in some acts as a cement to the siliceous crystals, but in others is in such great abundance that in burning them they become an imperfect porcelain and are then used to repair the roads, as at Chesterfield in Derbyshire; these are called argillaceous grit by Mr. Kirwan. In other places a calcareous matter cements the crystals together; and in other places the siliceous crystals lie in loose strata under the marl in the form of white sand; as at Normington about a mile from Derby.
The lowest beds of siliceous sand-stone produced from morasses seem to obtain their acid from the morass, and their calcareous base from the limestone on which it rests; These beds possess a siliceous cement, and from their greater purity and hardness are used for course grinding- stones and scyth stones, and are situated on the edges of limestone countries, having lost the other strata of coals, or clay, or iron, which were originally produced above them. Such are the sand-rocks incumbent on limestone near Matlock in Derbyshire. As these siliceous sand-rocks contain no marine productions scattered amongst them, they appear to have been elevated, torn to pieces, and many fragments of them scattered over the adjacent country by explosions, from fires within the morass from which they have been formed; and which dissipated every thing inflammable above and beneath them, except some stains of iron, with which they are in some places spotted. If these sand-rocks had been accumulated beneath the sea, and elevated along with the beds of limestone on which they rest, some vestiges of marine shells either in their siliceous or calcareous state must have been discerned amongst them.
2. SILICEOUS TREES.
In many of these sand-rocks are found the impressions of vegetable roots, which seem to have been the most unchangeable parts of the plant, as shells and shark's teeth are found in chalk-beds from their being the most unchangeable parts of the animal. In other instances the wood itself is penetrated, and whole trees converted into flint; specimens of which I have by me, from near Coventry, and from a gravel-pit in Shropshire near Child's Archal in the road to Drayton. Other polished specimens of vegetable flints abound in the cabinets of the curious, which evidently shew the concentric circles of woody fibres, and their interstices filled with whiter siliceous matter, with the branching off of the knots when cut horizontally, and the parallel lines of wood when cut longitudinally, with uncommon beauty and variety. Of these I possess some beautiful specimens, which were presented to me by the Earl of Uxbridge.
The colours of these siliceous vegetables are generally brown, from the iron, I suppose, or manganese, which induced them to crystallize or to fuse more easily. Some of the cracks of the wood in drying are filled with white flint or calcedony, and others of them remain hollow, lined with innumerable small crystals tinged with iron, which I suppose had a share in converting their calcareous matter into siliceous crystals, because the crystals called Peak-diamonds are always found bedded in an ochreous earth; and those called Bristol-stones are situated on limestone coloured with iron. Mr. F. French presented me with a congeries of siliceous crystals, which he gathered on the crater (as he supposes) of an extinguished volcano at Cromach Water in Cumberland. The crystals are about an inch high in the shape of dogtooth or calcareous spar, covered with a dark ferruginous matter. The bed on which they rest is about an inch in thickness, and is stained with iron on its undersurface. This curious fossil shews the transmutation of calcareous earth into siliceous, as much as the siliceous shells which abound in the cabinets of the curious. There may sometime be discovered in this age of science, a method of thus impregnating wood with liquid flint, which would produce pillars for the support, and tiles for the covering of houses, which would be uninflammable and endure as long as the earth beneath them.
That some siliceous productions have been in a fluid state without much heat at the time of their formation appears from the vegetable flints above described not having quite lost their organized appearance; from shells, and coralloids, and entrochi being converted into flint without loosing their form; from the bason of calcedony round Giesar in Iceland; and from the experiment of Mr. Bergman, who obtained thirteen regular formed crystals by suffering the powder of quartz to remain in a vessel with fluor acid for two years; these crystals were about the size of small peas, and were not so hard as quartz. Opusc. de Terrâ Siliceâ, p. 33. Mr. Achard procured both calcareous and siliceous crystals, one from calcareous earth, and the other from the earth of alum, both dissolved in water impregnated with fixed air; the water filtrating very slowly through a porous bottom of baked clay. See Journal de Physique, for January, 1778.
3. AGATES, ONYXES, SCOTS-PEBBLES.
In small cavities of these sand-rocks, I am informed, the beautiful siliceous nodules are found which are called Scot's-pebbles; and which on being cut in different directions take the names of agates, onyxes, sardonyxes, &c. according to the colours of the lines or strata which they exhibit. Some of the nodules are hollow and filled with crystals, others have a nucleus of less compact siliceous matter which is generally white, surrounded with many concentric strata coloured with iron, and other alternate strata of white agate or calcedony, sometimes to the number of thirty.