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perhaps, by a thin series of shales with the Culm-measures above. Evidence recently obtained by my colleague Mr. C. Reid at Chudleigh, seems to indicate the conformability there of the Culm-measures and Devonian Limestone. The main mass of the South Devon slates which underlies the Limestone, itself rests on Red Sandstones, similar in character to Old Red Sandstone. These beds are well shown at Cocking
ton, near Torquay.
Dr. Holl considers that in South Devon there is a complete unconformability between the base of the Culmmeasures and the underlying Devonian rocks. The beds of pale greenish and grey argillaceous slates, sometimes soft and silvery, and often veined with quartz, that extend from Callington, north of Plymouth, to Ashburton, he regards as the lower group of Devonian rocks; while the group of grey slaty rocks of Mount Edgcumbe, Kingsbridge, Dartmouth, and Totnes, constitute, in his opinion, an upper group.
It is only right to state that Mr. Jukes questions the unconformability of the Culm-measures and Devonian rocks in this southern area; the discordant positions can, he thinks, be accounted for by concealed dislocations and unrecognized contortions; and any one who has personally surveyed the country will agree with him that the faults and other disturbances are very numerous. Nor can the palæontological evidence be used for determining stratigraphical relations these must be first made out by long-continued, careful, and minute survey. Such a survey will, no doubt, prove the stratigraphical relations of the beds. Then the fossils may be placed in their true horizons. In the mean time every fact observed will help the solution of the problem, whether it be the record of some new section, or the determination of a suite of fossils from some definite stratum. Fossils collected are useless for purposes of classification unless
the exact locality be known: localities given simply as Newton, Chudleigh, or Torquay are valueless.
The following classification of the beds has been made by Mr. Etheridge :—
The age of the Petherwin limestone has been matter of much dispute: by some authorities it is regarded as Middle Devonian.
In Cornwall particularly but little is known of the stratigraphical relations of the slaty rocks. Mr. Pengelly has obtained from the lower beds Phyllolepis concentricus, and at Looe remains of Pteraspis. He also obtained a scale of Phyllolepis at the base of the cliff between Meadfoot Beach and the Thatcher Rock, Torbay.
Economic Products, &c., of Devonian Rocks.
The Killas, shillet, or clay-slate of Cornwall and Devon varies very much both in colour and in character in different
1 Mr. C. W. Peach (in 1844) discovered Fish-remains in these rocks.
places. It is the matrix of much of the mineral wealth of the district, including ores of tin, copper, silver, lead, and iron.
Of Cornish slates, those of De la Bole near Camelford, and Tintagel, are most highly valued. The De la Bole Quarries produce besides roofing-slates, flagstones used for pavingpurposes, tombstones, &c.
Devonian slate called Kingstone Stone has been quarried near Taunton, on the Quantock Hills. At Hestercombe the slate by contact with a syenitic dyke, has become metamorphosed and forms in places a hone-stone.
Grey micaceous slates, apparently very much altered, are met with at Salcombe and Bolt Head. Slate has been quarried at Tavistock, Kingsbridge, &c. At Tavistock, Devonshire oilstones were formerly obtained.
Rock Crystal (Cornish diamond) occurs in many parts of Cornwall, at Tintagel, De la Bole Slate Quarries, Carnbrea, &c.
THE term Carboniferous is applied to a great system of formations, which are intimately linked together, and which in our country are the great coal-producing rocks. It is, however, only in the upper portion or in the Coal-Measures proper that coal is usually worked.
Although connected with the Old Red Sandstone by gradations that forbid any sharp line of demarcation being drawn, yet the organic remains of the Carboniferous rocks are distinct from those of the Old Red Sandstone, and they were evidently deposited under very different physical conditions. As they comprise a mass of strata of great thickness, it is well to mark the distinction by classing them as a different system under the comprehensive term of Carboniferous.
The Carboniferous rocks were mostly formed in the sea and not far from the land, for the remains of terrestrial vegetation occur in them at all horizons, and hence the period has sometimes been termed the Phytozoic Period, or Age of Plants.
Very marked lithological changes occur in the beds when traced across England. The Lower Limestone Shales of the south are scarcely represented as a distinct group in the north; the Mountain Limestone of the south and centre loses its marked calcareous character in the far north, and becomes a series of limestones and sandstones with coal; the Millstone Grit exhibits many modifications in different localities; the Yoredale Rocks scarcely noticed in the south attain great
importance in the north; while the Coal-Measures, perhaps the most persistent in type of the series, is at least a very unprofitable member in Devonshire.
The following are the principal local divisions that have been made in the Carboniferous rocks :
1. Northumberland and Durham.
Coal-Measures (1,500 to 2,000 feet).
Millstone Grit (400 feet).
Upper coal-measures with thin coals.
Alternations of grit and shale.
Yoredale rocks and Carbon- (Alternations of limestone, coal, shale, grit, iferous Limestone series.1 and sandstone.
White and grey sandstone with greenish grey shales, cement-stones, and limestones.
The Yoredale Rocks are from 500 to 540 feet in thickness; and the lower Carboniferous beds from 1,100 to 2,000 feet.
2. Lancashire, Yorkshire, and Derbyshire.
(2,000 to 7,000 feet).
Upper coal-measures with limestores and thin coals. (Lancashire only.)
Middle coal-measures with thick coals.
Lower coal-measures or Gannister2 beds with
1 Mr. G. A. Lebour has suggested the term 'Bernician Series' for this group, because no Yoredale rocks proper, and no Scar Limestones proper, can be shown to exist in it.
2 The term 'Gannister' or 'Calliard' is used to designate a peculiarly hard siliceous bed, forming the floor of some of the coals of this series. (Hull.) It is often full of the rootlets named Stigmaria ficoides.