specks of white calcareous spar, or, as has been supposed, of steatite. Nearer the surface, it is still more porous, light and ruddy; and the many white spots, now enlarged, are filled with a soft chalky substance."* The pores of some varieties, which are in a state of decomposition, are filled with a beautifully green malachite; in others, the cavities apparently contain lumps of a black powder, which, on examination by the microscope, with powerful glasses, is discovered to be a mixture of ferruginous crystals and an ochrey earth, strewed with a few specks of chalk. "Out of the schistus near Crediton arises a compact lava, of a purple color, with large crystals of felspar, and numerous crystals of pellucid quartz and black mica, the cavities containing farinaceous steatite: it does not effervesce with acids. In this specimen it is remarkable, that the crystals of felspar have cavities in them, and are filled also with the steatite, like the rest of the stone.+" Specimens of basaltes have been obtained at South-Bovey, Bishop-Steignton, and near Crediton. Granite, called also moor-stone, as in Cornwall, is met with in various places, but particularly on Dartmoor, where the moun tains commence which extend into that county. It generally lies in vast irregular masses; and is here found in great variety, both as to texture and color. Specimens of the red granite are exceedingly beautiful when well polished. On exposure to the atmosphere, it becomes extremely hard; but when first raised, may be worked with less difficulty. The most remarkable of the Inflammable substances discovered in Devonshire, is the Bovey coal, the origin of which has occasioned considerable discussion among geologists. It is obtained in the extensive flat called Bovey-Heathfield, which appears to have been formerly covered by the tide, and is supposed to be lower than the level of the sea. Its strata run nine miles to the southward, through the Heathfield, by Knighton, Teigngrace, and Newton Marshes, to Abbots-Kerswell, generally keeping to the west of the beds of potter's clay, which range through various parts of the VOL. IV. Heathfield, C History of Exeter, p. 80. + Polwhele's Devon, Vol. I. p. 63. Heathfield, and sometimes crossing them. "The uppermost of the strata rises to within a foot of the surface, under a sharp white sand, intermixed with an ash-colored clay, and under-lies to the south, about twenty inches in a fathom: the perpendicular depth of these strata, including the beds of clay with which they are mixed, is about seventy feet. The strata of coal near the surface are from eighteen inches to four feet thick, and are separated by beds of a brownish clay, nearly of the same dimensions, but diminishing in thickness downward, in proportion as the strata of coal grow larger; and both are more compact and solid in the lower beds. The lowermost stratum of coal is sixteen feet thick: it lies on a bed of clay, under which is a sharp green sand, not unlike sca-sand, seventeen feet thick; and under that, a bed of hard coarse clay, which has not been bored through."* From the thick bed of sand rises water of a vivid green color, which is said to abound in sulphur and vitriol, and is as warm as some of the Bath springs. In some of the beds of clay are small and narrow veins of coal, shooting through and forming impressions like reeds and grass. The coal that is.taken up for use, is obtained from an extensive open mine (having an easy descent for horses to bring up the produce) at the west end of South-Bovey Town. Its peculiar properties are thus described in Polwhele's History of Devonshire. "Though the substance and quality of the Bovey coal, in its several strata, be much alike, and all indiscriminately used for the same purposes, yet there is some difference in the color, form and texture of the several veins. The exterior parts, which lie nearest to the clay, have a greater mixture of earth, and are generally of a dark brown or chocolate color: some of them appear like a mass of coal and earth mixed: others have a laminous texture; but the lamine run in such oblique, wavy, and undulatory forms, that they bear a strong resemblance to the roots of trees. There are other veins of this coal, which lie more in the centre of the strata, and abound most in the lowest and thickest bed, the substance of which is more compact and solid; these are as black, and almost as heavy, * Letter of the late Dean Milles to the Earl of Macclesfield, 1760. as At as pit-coal. They do not so easily divide into laminæ, and seem to be more strongly impregnated with bitumen. They are distinguished by the name of stone-coal, and the fire of them is more strong and lasting than that of other veins. But the most curious vein in these strata is that called the wood-coal; which is sometimes of a chocolate color, and sometimes of a shining black: the former sort seems to be less impregnated with bitumen, is not so solid and heavy as the latter, and has more the appearance of wood. It lies in straight and even veins, and is frequently dug in pieces of three or four feet long; and, with proper care, might be obtained of a much greater length. Other pieces of the same kind are found lying upon them in all directions, but without the least intermixture of earth, or any interstice, except some small crevices, by which the pieces are divided from each other. When it is first dug, and moist, the thin pieces will divide like horn; but when dry, it loses its elasticity, and becomes short and crisp. all times it is easily separated into very thin laminæ, or splinters, especially if it lie exposed to the heat of the sun, which, like the fire, makes it crackle, separate, and fall to pieces. This fossil consists of a number of lamina, or very thin plates, lying upon each other horizontally, in which small protuberances sometimes appear like the knots of trees; but they are only mineral nuclei, which occasion this interruption in the course of the lamina; and pieces of spar have been sometimes found in the middle of this wood-coal. Though the texture of this coal is laminated, yet it does not appear to have any of those fibrous intersections, which are observed in the grain of all wood. It easily breaks transversely; and the separated parts, instead of being rugged and uneven, are generally smooth and shining, and even the course of the laminæ is hardly discernible. The fire made by this coal, is more or less strong and lasting, according to its different veins: those which lie nearest to the clay, having a greater mixture of earth, burn heavily; and leave a large quantity of brownish ashes. The wood-coal is said to make as strong a fire as oaken billets, especially if it be set on edge; that the fire, as it ascends, may insinuate between, and separate the lamina. But the heat of the stone-coal is accounted the strongest, though not sufficiently intense for the mines. When this coal is put into the fire, it crackles, and separates into lamina, burns for some time with a heavy flame, becomes red-hot, and gradually consumes to light white ashes.* Though the transverse crevices made in it by the fire, give it the external appearance of a wooden brand, yet, if quenched when red-hot, the unconsumed part seems to be almost as smooth and solid as when first put into the fire. The thick heavy smoke which arises from this coal when burning, is very fetid and disagreeable; entirely different from the aromatic scent of the bituminous loam which is found adhering to it, but much resembling that of the asphaltum, or bitumen of the Red Sea. That part of the clay which lies nearest to the coal, seems to partake of its nature, being somewhat of a laminous texture, and in a small degree inflammable: and among this clay, but adhering to the veins of coal, are found lumps of a bright yellow loam, extremely light, and so saturated with petrolium, that they burn like scaling-wax, emitting a very agreeable and aromatic smell." The basis of the Bovey coal is generally supposed to be vast assemblages of trees, that have, in various and distant ages, been washed by torrents from the neighbouring hills; and on which, from time to time, intervening beds of clay have been deposited. This opinion is corroborated by the situation of the Heathfield, which at some period was probably a morass, and is almost encompassed by the secondary hills that undulate at the feet of Dartmoor and of Haldon; and likewise, by the lightness and appearance of the wood-coal; the nuclei found in it, and the laminæ being taken up in all directions, as if formed of trees laid confusedly across each other. The argument for its vegetable origin may be still further strengthened by comparing it with the Surturbrand of Iceland, and the Piligno of the Italians, which are unquestionably fossil wood; and resemble the Bovey coal too nearly to admit the supposition that the origin of the latter can be different. The accurate Kirwan observes, that it consists of wood penetrated with petrol, The small of the Bovey coal, when thrown on a heap, and exposed to the weather, will take fire of itself. petrol, or bitumen, and frequently containing pyrites, alum, and Its specific gravity is from 1.4. to 1.558: its proportion vitriol. Pyrites is obtained in various parts of the county, and has not unfrequently been found in globular balls of different sizes. A great number were met with a few years ago in the schistus, near Chudleigh, lying at some distance from each other. Several of them are in the cabinet of P. Rashleigh, Esq. of Menabilly, who has described them as follows. "The balls which I have, weigh from fifteen drams to five ounces. They are nearly circular, and resemble military bullets of the same weight: the outward coat is of a brown rusty color, composed of very minute angular crystals, either triangular, or quadrangular: the inside is a very solid shining substance of sulphur and iron, not radiated, like the pyrites found in chalk-pits; they are embedded in a black hardened clay, C 3 "The production of coal from morasses," it is observed by Dr. Darwin, in his Botanic Garden, " is evinced from the vegetable matters frequently found in them, and in the strata over them; as fern leaves in nodules of ironore; and from the bog shells, or fresh-water muscles, sometimes found over. them; and is further proved from some parts of these beds being only in part transformed to coal; and the other part still retaining not only the form, but the properties of wood. Specimens are not unfrequent in the cabinets of the curíous, procured from Bovey coal, near Exeter, and other places." The particu lar species of wood of which the Bovey coal was probably formed, is supposed to be the pine; and a specimen, with the bark remaining, is now said to be in the possession of Dr. Cornish, of Totness. This opinion will be in part con firmed from the following information, obligingly communicated by Mr. John Pering, of Rockford. "On examining the appearance of the ground about one hundred yards from the pits," observes this gentleman, "I was struck with what appeared to me a very material circumstance, and of which it seemed strange I had never before heard the slightest account, either in conversation with many who had been purposely to examine the place as a curiosity, or in any written or printed account. The fact is, I found, just level with the Heathfield, numerous stumps of trees, which appeared to have formerly belonged to bodies of immense size. They were not dug up, but fixed with their roots in their natural position. Their appearance was much jagged; but I do not recollect any evident marks of the saw. I cut some pieces, a few of which are now before me: they evidently have been cut with the axe; and in color, lightness, and texture, strongly resemble deal. If so, the species of tree was probably the Pinus Sylvestris, or Scotch Fir." |