in a particularly striking form on the coasts of Bridesbay, Pembrokeshire, near Littlehaven. Gen. erally speaking where solid masses of limestone and sandstone associated with the coal are raised up in high angles, but still placed in nearly regular planes, the more tender argillaceous or slate-clay beds are usually folded, or as it were crumpled together. The Mendip hills and adjacent collieries in Somersetshire afford good illustrations of this fact, which strongly suggests the idea of a mechanical force having elevated the more solid rocks in mass; while the more pliant materials, yielding to its lateral pressure, have become irregularly contorted. These phenomena cannot be ascribed to any intrinsic agency like crystallization; for they appear common to all rocks, even those most decidedly mechanical in their structure. They are equally observable in the most recent members of the oolitic series in the Isle of Purbeck.

The faults, slips, or dislocations of the coal-fields, are irresistible evidences of their having been affected by violent mechanical convulsions, subsequently to their original formation. These fissures which traverse the strata in so many directions, have occasioned the downfall, and upheaving of the two portions which they disjoin. Hence when the miner advancing in the line of his seam, comes to a fissure, he no longer finds the coal on the same level; the corresponding part of it, is one, two, three, or more yards lower or higher according to the force of dislocation. These fissures are frequently filled with fragments of sandstone, and the other substances of the coal formation, when they

COAL FAULTS IN THE BRISTOL BASIN. 159

are called faults. Their magnitude is often considerable, amounting sometimes to 100 yards in thickness. At other times the seams of coal are not deranged from their position, but the portions contiguous to the faults are as it were contorted or broken. It is obvious hrow interesting to the miner, the knowledge of these dislocations must be, of which a great many are occasionally crowded into a narrow space. They are sometimes called traps, from their resemblance to a step, so named in the northern tongues. The direction of the fissures is commonly vertical.

Paulton Hill.

20

2

70

Paulton Church.

Faringdon Seams. The above figure is part of a section of the Bristol coal basin, from N.N.W. to S.S.E. The fault of 70 fathoms, on the left, is near Midsummer Norton; that at the right, is near High Littleton. Paulton hill, to the left of the 20 fathom fault, is topped with oolite, beneath which, is a bed of lias, then the newer red sandstone, and finally, the pennant and coal seams. The wood-engraving is accurately copied from a section accompanying the Rev. Dr. Buckland's and Mr. Conybeare's excellent paper on the south-western coal district of England.-Geological Transactions, New Series, Vol. I. p. 210.

The coal-measures, especially in Great Britain, are also sometimes cut across by great veins or dykes of a basaltic nature. At Newcastle these are very numerous, and their size is sometimes prodigiously great. They extend through a great many

ness.

miles of country, with a thickness of ten, twenty, or even fifty yards. Their colour is blackish-green; they are compact, and sometimes amygdaloidal, having almond-like concretions interspersed in their substance. In their proximity, the coal is carbonised, assuming a grey colour, and a reedy structure, the sulphur is sublimed out of the pyrites, and the sandstone has acquired considerable hardThese changes extend to several yards distance, even to twenty in some situations. Occasionally these veins seem as if composed of two portions separated by a middle space, a few yards wide, which is filled with the materials of the coal formation, more or less altered. In one of these interstices, galena has been found. The two portions of the same coal-seam remain sometimes on the same plane, on either side of the vein; but at other times, the deviation is very great, being nearly 200 yards alongside of one dyke in Northumberland; that being the most remarkable one of the district.

Basaltic dykes traverse the subjacent formations of limestone and millstone-grit, as well as the coal measures themselves. They may be seen indeed penetrating up into rocks of much later date. Limestone is often rendered highly crystalline and unfit for burning into lime, from the influence of this rock, as happens to the two undermost strata at Wratchiff crag in Northumberland. Slate-clay, or shale, is turned into a substance like flinty slate or porcelain jasper. This is the case with the stratum lying immediately beneath the upper bed of basalt at Wratchiff crag; and the coal is invariably charred, into blind coal or coak, when in con

REMARKABLE TRAP DYKES NEAR NEWCASTLE. 161

town.

tact with it. The sandstone on which it sometimes reposes, is changed for some depth to a brickred colour. The most considerable dyke in the neighbourhood of Newcastle, is that which passes through the Coley-hill, about four miles west of the The dyke is vertical, and 24 feet thick. The basalt composing it, lies in detached masses, which are coated with yellow ochre. The removal of these brings to view thin layers of indurated clay, with which the fissure is lined, which breaking into small quadrangular prisms, are used by the country people for whetstones. In this argillaceous substance, clay-ironstone impressed with the figure of ferns is very abundant.

The upper seam of coal occurs here at about 35 feet beneath the surface, and where, in contact with the dyke, is completely charred, forming an ash-gray porous mass, which breaks into small columnar concretions, exactly resembling the coak obtained by baking coal in close iron cylinders in the process of distilling tar. Calcareous spar and sulphur are disseminated through the pores of this substance.*

To the east-south-east of the Coley-hill dyke in the line of its direction, a vein is found traversing Walker Colliery, and crossing the Tyne at Walker. This dyke is well defined. It occasions no alteration in the level of the coal strata, and its depth is unknown. It has been cut through by horizontal tunnels at four places. On each side of the dyke, the coal is converted into coak, which on one side, in some places, was found to be 18 feet thick, and on the opposite side upwards of nine feet. A firm, hard, and unbroken vein of basalt, on an average, about 13 feet thick, was in immediate contact with the coak on each side; and between these two veins lay nodules of basalt and sandstone, upwards of nine feet in thickness, imbedded in a cement of blue slate.

A dyke called the Cockfield dyke, which is 17 feet wide, underlies to

P

In Derbyshire, the Basaltic beds and veins are called Toadstone from their colour, and in Warwickshire, Rowleyrag, from a group of hills formed of it at the village of Rowley, near Birmingham.

The trap often occurs as at Rowley, in overlying

the south, and throws up the coal measures on that side 18 feet. The low main-coal contiguous to the basalt, is only nine inches thick, but enlarges to 6 feet at the distance of 150 feet from it. The coal is reduced to a cinder, and the sulphur is sublimed from the pyrites near to the dyke.

A dyke is seen on the banks of the Tees, a little below Yarm. It there passes into the newer red sandstone, and continuing its course in the same direction, is well known to traverse the north-eastern part of Yorkshire, near the still more recent formations of lias, and the sandstone of the inferior oolite, in the eastern Moorlands, in its way to the German Ocean. This dyke is highly interesting from its great length, and from the evidence which it affords by thus penetrating later rocks, that it must have owed its origin to eruptive forces, in action at a period long subsequent to the formation of the coal; a proof which yields a strong analogical presumption that the other dykes of the coalmeasures are likewise subsequent, and not contemporaneous phenomena.

These circumstances render the course of this dyke, through the more recent formations, a matter of much geological importance. It may be traced from Berwick on the Tees in an easterly direction, near the villages of Stanton, Newby, Nanthorp, and Ayton. At Langbath ridge a quarry is worked in it; it passes south of the remarkable hill called Roseberry Topping, near Stokesly, and thence by Lansdale to Kildale. It may be seen on the surface nearly all the way in the above track. From Kildale it passes to Denbigh-dale end, and through the village of Egton-bridge, and hence over Leace ridge through Gothland, crossing the turnpike road from Whitby to Pickering, near the seven-milestone at a place called Sillon-cross on a high moor. Mr. Bakewell examined it at this place, where it is quarried for the roads, and is about 10 yards wide. From this place it may be traced from Bleahill to Horwood dale, in a line towards the sea, near which it is covered with alluvial soil; there can be no doubt, however, that it extends into the German Ocean. It is a dark grayish-brown basalt, which turns brown on exposure to the atmosphere, and is the chief material for making roads in the district called Cleveland. The dyke enters the lias near Nanthorp, and the sand of the inferior oolite near Roseberry Topping.