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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 whittening' - the

* See note on p. 111.

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Sheffield whittle from the earliest periods being in all probability ground on this stone. Wickersley stones are obtained about nine miles from Sheffield, and are much used by the cutlers for grinding.'

In the lower beds of the Coal-measures near Bradford, iron-pyrites has been worked for the manufacture of sulphuric acid and sulphate of iron.

The Clay-ironstone of the Coal-measures yields much of the iron in this country. Petroleum springs have been met with in the Coal-measures near Broseley ; one called the • Burning Well’existed about a century ago.

Supply and Duration of Coal. The subject of our supply of coal has been exhaustively treated in the Reports of the Royal Coal Commission. It appears that there is a quantity of Coal in England and Wales in the visible Coalfields, available at a depth not exceeding 4,000 feet, equal to upwards of eighty thousand millions of tons; whilst the amount of coal similarly available beneath the Permian and newer strata, is estimated at about fifty-six thousand millions of tons. The probable duration of our supply is variously estimated, but that it is considered good, at depths readily accessible, for at least 276 years is satisfactory.

| Hunt and Rudler, Descriptive Guide to the Museum of Practical Geology.

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SECTION II.

SECONDARY OR MESOZOIC.

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CHAPTER V.

POIKILITIC.

COMPARED with the Palæozoic rocks, the Secondary strata form a very distinct group. The beds, although well consolidated, are yet, on the whole, free from the striking signs of alteration or metamorphism which are so largely present in the older rocks. The lines of stratification are marked and easily recognizable, and the beds have suffered but little from the effects of great disturbance which are so conspicuous in the deposits of earlier times.

Although in some localities an apparent passage has been observed of the Permian beds into Coal-measures, yet on the whole the Secondary strata lie unconformably upon the denuded Palæozoic rocks. Lyell has well remarked,

that nowhere have geologists found more difficulty in drawing a line of separation than between the Secondary and Primary series,' for indeed there is no great break between them.

The Secondary rocks are characterized by Marsupial Mammalia ; by numerous remains of Reptilia, such as the Ichthyosaurus and Plesiosaurus; by Mollusca, of the genera Ammonites, Belemnites, Trigonia, Rhynchonella, and Terebratula; by Corals, and by Cycadeiform plants.

ܪ.

POIKILITIC.

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The Permian and Triassic beds have been very differently classed by geologists, because the strata taken as a whole form a connecting link between the Palæozoic and Mesozoic periods.

Thus, whilst the fauna and flora of the Permian rocks unite them more closely to the Palæozoic rocks than to the Trias, in their lithological characters and in the evidences of their method of formation, the two groups seem inseparably connected, and with them the Rhætic or Penarth beds may very conveniently be classed.

On geological maps the group forms a conspicuous band, stretching across England from the mouth of the Tees to the mouth of the Exe, with a branch running to the mouth of the Mersey : thus marking off the Palæozoic ground of the north of England, of Wales, and of the S.W. of England, from the Secondary and Tertiary tracts, which lie to the south-east.

The Permian and Triassic rocks, from their prevailing red colour, are equally prominent, and the red ground and “red rocks’ have given many a name to our hamlets, villages, and towns, such as Rotherham, Retford, Radford, Radcliffe, Radstock, &c.

The term New Red Sandstone was originally used to distinguish these strata from the Old Red Sandstone.

In their general lithological characters there is a marked similarity throughout the Permian and Triassic series, consisting as they do of red sandstones, conglomerates, and marls, with occasional beds of limestone. Originally, the whole of these rocks were classed as New Red Sandstone, and the name Poikilitic' (meaning variegated), subsequently suggested by Conybeare as an equivalent term, is one that possesses many advantages to recommend it.

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