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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.
1 Hunt and Rudler, Descriptive Guide to the Museum of Practical Geology.
SECONDARY OR MESOZOIC.
COMPARED with the Paleozoic 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.
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.
On physical grounds Sedgwick classed the Permian series with the Trias.
There are instances in Lancashire, Cheshire, and Derbyshire, where a kind of gradation appears between the Coalmeasures and the Permian sandstones, which are locally conformed to them; near Coventry the Permian beds appear conformable to the Coal-measures, and to be overlaid unconformably by the Trias; and in East Yorkshire there is considerable similarity between the beds at the base of the Permian series and those at the top of the Coal-measures, and the beds have been differently classified in places by geologists.
Professor Hull, however, believes that there is an unconformability between the Permian and Carboniferous rocks in Lancashire and Yorkshire.
It is considered by some observers that the different members of the Permian formation are not strictly conformable to one another. A most decided instance of unconformability is stated to occur in the railway-cutting at Tadcaster. The Middle Marl has there thinned away to a mere seam, so that the Upper Limestone rests almost directly on the Lower, and at the base of the former there is a thin bed of gravel formed of Lower Limestone pebbles.' '
In isolated masses, as might be expected, it is not always easy to distinguish between the Permian and Triassic beds. Thus certain red sandstones in Lancashire, referred to the Permian period by Mr. Binney, have been considered as Bunter by Professor Hull; although the latter admits that, 'isolated as the beds are here, there can be no certainty regarding their age.' In some parts of Yorkshire, the Permian seems to pass upwards into the Trias.
The Rev. A. Irving has remarked that there are clear signs of continuous deposition of the Permian and Lower 1 Explanation of Quarter Sheet 93 S.W. (Geol. Survey).
Bunter rocks in Nottinghamshire; and, so far as that area is concerned, the stratigraphical data seem to point to the conclusion that the Permian and Bunter are but portions of one great unbroken sequence of rocks.
In treating of the physical history of the Permian and Triassic rocks of Great Britain, Professor Ramsay has pointed out that the beds were deposited in great inland lakes for the most part salt. One objection might be taken to this theory, inasmuch as the organic remains of the Magnesian Limestone are truly marine types. Professor Ramsay, however, observes that these are wonderfully restricted when compared with those of Carboniferous times, and in the poverty and dwarfing of the forms the fossils of this rock may be compared with the still less numerous fauna of the Caspian; so that he considers they might have lived similarly in a large inland salt lake which had previously been connected with the open ocean.
MAGNESIAN LIMESTONE SERIES.
The name Permian was proposed by Murchison in 1841, from the ancient kingdom of Permia, in Russia.
The Permian rocks consist of red sandstones, conglomerates, marls, and magnesian limestones, of which the following series is generally established:
or Magnesian Limestone Series
Red Sandstone and
stone. Marl slate.
Lower Magnesian Limestone.
Lower Red Sandstone, Marl, Breccia, and Conglo