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Limestone and black shale, with Posidonomya, Gonia
tites, &c. Posidonomya (Posidonia) limestone of Swimbridge and Venn.
Black shale group.
(2) The South Devon strata about Trescott and Lew Trenchard have been thus divided:
Gritstone group of Central Devon.
Upper shale group-dark shales, carbonaceous grits and shales, equal to the Coddon Hill series. Calcareous group-limestone of dark colour, and irregular bedding, with shales (Posidonomya).
Lower shale group, with few fossils. (No slaty cleavage.)
The Culm-measure series is characterized by beds of greenish-grey grit, black-looking shales, sometimes cleaved, and occasional beds of quartzose conglomerate. The beds of grit are sometimes sufficiently developed to be useful for building purposes, as near Chudleigh, while flagstone beds are worked near Launceston. The Grinshill freestone has been worked near Bideford.
The researches of Sedgwick and Murchison tend to show that although this overlying series is in mineral aspect as much unlike the Carboniferous strata of most other parts of Britain, as the rocks of North Devon are unlike the ordinary Old Red Sandstone of England and Scotland, yet there are proofs by fossils, besides the analogy with Pembrokeshire, that the black limestones of Swimbridge and Venn, &c. with Posidonomyæ represent, on a miniature scale, a part of the Carboniferous Limestone; that the next series of white grit and sandstone of Coddon Hill, &c. stands in the place of the Millstone Grit; and that the overlying courses of culm, grits and shales, with many remains of plants, are the equivalents of the lower coal-bearing strata. At the same time it must
not be overlooked that beds lithologically identical with Old Red Sandstone occur at the base of the Devonian series in North and South Devon, and that the upper portion of this series may represent some of the Lower Carboniferous rocks.
The Black or Culm-limestones occur at Bampton, Holcombe Rogus, Canon Leigh, and West Leigh, where the beds are often very much disturbed and even violently contorted. They are also met with at Drewsteignton, and near Launceston.
The Culm-limestone according to Sedgwick and Murchison often appears to come to an edge or to pass into calcareous shale.
In the large quarries seldom more than a third or fourth part can be burnt for lime: the alternating bands of dark indurated shale are used for flagstones or coping-stones.
The base of the Culm-measures, particularly between Barnstaple and South Molton, is noted for the occurrence of Wavellite (hydrous phosphate of alumina), which exhibits its radiating crystals on the broken transverse joints of the slaty rocks: hence the term 'Wavellite schists.'
In places near the granite of Dartmoor the rocks are much metamorphosed, and beds of indurated black slate pass into lydian stone; near Bampton the shales become indurated and cherty.
North of Boscastle the beds are traversed by large quartz veins and are violently contorted. According to Sedgwick and Murchison some of the shales are calcareous, and contain masses of a variety of Rottenstone like that in the limestoneshales of Derbyshire.
Mines of Lead, Iron, and Manganese have been opened in different places in the Culm-measures.
The beds of Culm stretch across the country from Barnstaple Bay to Bideford towards Chittlehampton, and they
have been worked at different periods, although never with
any great profit.
Murchison has observed that the coal-field which is bituminous in Monmouth, Glamorgan, and Caermarthen, becomes anthracitic in Pembroke, where the stone-coal series, much disturbed and broken, differs from that of Devon only in being much more productive. Possibly some of the culm-strata of Devon, devoid as they are of any workable coal, may yield bituminous products by the application of heat. Some of the culm-beds of Devon may be considered subordinate to the Millstone grit; but most of the culm overlies that rock, and is simply the equivalent of the culm of Pembrokeshire.
Economic Products, &c., of Coal-Measures.
The Coal-measures form generally an unproductive soil.
The flags and sandstones are extensively quarried for building and paving purposes, and for firestone. Amongst these may be mentioned the Heddon and Kenton Freestone, Newcastle; Catlow stone (laminated flagstone) and Hapton stone (flagstone), Burnley, Rochdale flagstone; Upholland flags (lower coal-measures), used also for grindstones and scythe-stones, Orrel, Billinge Hill and Upholland, Lancashire; Peel stone, near Bolton; Thwaites Delf white stone, Wigan; Rushy Park roof-stone, Rainhill and St. Helens. Clay Wood stone,* Slatestone rock * (roofingslates, paving), Handsworth Bluestone, Brincliffe Blue stone, Brincliffe Edge, used for flags, gravestones, for whitening stones, &c. (lower coalmeasures), Lidgate rock, Manor stone (building and flags), Grimesthorpe, Wadsley stone, near Sheffield; Potternewton stone, near Leeds; Yorkshire paving-stone, Bretton quarry, Halifax (lower coal-measures); and the Shepley stone (flags) near Huddersfield.
Near Bradford the Adwalton and Birstal rock, and the Bradford and Elland flagstone, are of economic importance. Some flags of a size twelve feet square have been obtained: thin beds are used for roofing purposes.
• These are rocks in the Carboniferous Series, but doubtfully referred to the Coal-Measures.
The following rocks are also of Carboniferous age:-Shaw Lane stone,* Belper; Wingerworth stone, near Chesterfield; Freebirch stone, near Chesterfield; Gornal sandstone? Sedgeley, S. Staffordshire.
The Dean Forest stone is used for building-purposes and grindstones; the Black Pins rock is worked in Ebbw Vale (Lower Coal-measures).
The Clays of the Coal-Measures are used in the manufacture of bricks and tiles, earthenware and pottery: the clays of Stourbridge are noted for the manufacture of firebricks, so are some at Newcastle, Bradford, &c.
In Shropshire a red marl from the Coal-measures is employed in the manufacture of encaustic tiles.
In the South Wales Coal-field, near Merthyr-Tydvil, Hatchettine, or mineral tallow, a hydro-carbon, is found.
Near Bradford the Gannister or Calliard stone has been ground to a fine sand to be used in the casting of iron and brass.
Grindstones sometimes called Newcastle grindstones are formed from sandstones from the coal-districts of Northumberland, Durham, Yorkshire, and Lancashire.
At Bilston, in Staffordshire, a peculiar sandstone is found lying above the coal, finer than the above, and of a very sharp nature. This is quarried entirely for the Bilston grindstones, which are of great excellence.
The carpenter's millstone is a hard and close variety of the Yorkshire sandstones. The northern counties yield several varieties of grindstones, which are much in request for different descriptions of work. Yorkshire grit, for example, is used for polishing marble and the copper plates for engravers. The Sheffield grindstone is a hard and coarse. stone used for common purposes; it is found at Ardsley, 14 miles north of Sheffield. The Sheffield blue stone is a fine-grained stone, used for finishing fine goods. The act of grinding on ' a blue stone is termed whitening' - the See note on p. 111.