Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/66

Rh COAL [COAL-FIELDS. neighbourhood of Sundcrland, more especially at Ryhope and Monkwearmouth which are worked at a depth of about 1850 feet to a short distance from the shore. At these points the coals are nearly flat, but at Harton, near Shields, they rise to the eastward, proving that the centre of the basin has there been passed. The best estimate gives 1 1 feet of coal, and about 1 G square miles of area, as the probable extent of this submarine portion of the coal measures. The character of the coal produced varies in the different parts of the basin. The southern and western districts adjoining Bishop Auckland and Ferry hill produce a strongly caking coal, which is chiefly employed in the manufacture of a pure and dense coke for use in the Cleveland and Cumberland iron works, a considerable amount being also exported for foundry use. The central district, adjoining Newcastle and Sundcrland, produces the best class of house coal, known in London under the name of Wallsend, from the pits on the Tyne where it was originally mined, .which were close to the eastern termination of the wall built by the Romans to protect the country between the Tyne and the Solvvay from the incursions of the Picts. These collieries have been long since abandoned, but the name is still given in the London market to the best Durham house coals, and even to much that has been produced in other places, as indicating a coal of superlative excellence. The great merit of Wallsend coal is in its small proportion of ash, which also, being dark -coloured, is not so obtrusive on the hearth as the white ash generally characteristic of the Midland coals. The strongly caking property, and the large amount of gas given out in burning, tend to produce a bright and enduring fire. In. the district north of the Tyne the produce is principally steam coal, which is known as Hartley coal, being named after one of the principal collieries. It is largely used for sea-going steamers, and was lately in use in the Royal Navy mixed with South Wales coal, a combination which was supposed to give a higher evaporating value in raising steam than when either class was burnt alone. Although of a lower calorific power, and making more smoke than South Wales coal, the north country coal deteriorates less rapidly than the former when stored in hot climates. There are two small coal-fields in the mountain limestone district of the Tyne near Hexham, and another on the Solway at Can- nobie; these are, however, of small importance. The Cumberland field extends along the coast of the South Irish Channel from Saint Bees northward for 15 miles to Maryport, where it turns eastward for about 1 7 miles, and is exposed with constantly diminishing breadth until it disappears under the Permian rocks of the Vale of Eden. The greatest breadth is about 5 miles at White- haven and Workington, but. as in Northumberland and Durham, the beds dip and the coals have been worked below the sea to a distance of 1 J miles from the shore or 2 miles from the pit. The total thickness of the mea- sitres is 1500 feet, with three workable seams. The pro duce is largely consumed within the district, a considerable portion of the export being to Belfast and other Irish ports. Denbigh- The coal measures of North Staffordshire and Lanca shire reappear on the western side of the plain of Cheshire coal -fields. in the coal - nelds of Denbighshire and Flintshire, which form a nearly continuous tract from the neighbourhood of Oswestry through Ruabon and Wrexham to the mouth of the Dee, and along the Welsh coast near Mostyn. The .separation between them is formed by a slight roll in the mountain limestone near Gresford, corresponding to that dividing the two coal-fields on the eastern out-crop. The Denbighshire field is about 18 miles long, having 7 seams, together from 26 to 30 feet in thickness. The principal workings are near lluabon, where there are several large coal collieries producing a much esteemed house coal. The Flintshire field is about 15 miles long. The greatest breadth is in the neighbourhood of Mold, whence it nar rows in a N.W. direction, being covered by the estuary of the Dee. At Mostyn coal has been extensively worked under the river, but great difficulty was experienced in keeping the mines clear of water. The details of the measures in this district have not been fully worked out, but the southern portion is the most valuable. The higher measures contain six seams, including some valuable beds of caunel, the total being about 28 feet. In the northern district bordering the Dee the beds are much disturbed by faults, but the deeper coals are said to be of good quality. The basin formed by the North Wales, Lancashire, and North Staffordshire coal-field is probably the most extensive tract of coal measures in the country, as it may be assumed to extend under the overlying Triassic strata under the Dee and the Mersey to South Lancashire and across the plain of Cheshire, an area of 800 to 1000 square miles. Much of this, however, is far beyond workable limits, the depth to the top of the coal measures being estimated at 10,000 feet below the surface at the point of greatest depression. The area within the limits of 4000 feet below the surface, which has been assumed as a possible maximum working depth, may be seen by refer ence to Plate I. There is a small coal-field in the Island of Anglesea, which is interesting for its geological peculiarities, but it is of very small economic value. The Somersetshire coal-field appears at the surface in the form of several disconnected patches, the largest of which extends northward of Bristol for about 12 miles, while the remainder stretches southward for about the same distance to the Mendip hills. The Carboniferous lime stone is seen at many places along the western flank, but the connection is generally hidden by a peculiar modifi cation of the New Red Sandstone known as the Dolomitic Conglomerate, which overlaps both formations indifferently. Towards the east the measures are further obscured by the overlap of the lias and oolitic rocks, this being the only field in which such an overlap takes place in England. The exposed area of the coal measures is only about 14 square miles, but it is estimated that they extend over 238 square miles, the remainder being concealed by overlying strata. The character of the measures is similar to those of South Wales and Dean Forest, namely an upper and lower pro ductive series separated by a nearly barren mass of Pen nant sandstones. The sections, which vary very consider ably, are summarized by Prestwich as follows: Upper series 2600 feet thick, with 16 seams, together 26 ft. 10 in. thick. Pennant sandstone, 2500 to SOOO ,, 4 50 Lower series, 2SUO 26 C6 6 Together, 7900 to 8400 46 93 4 The disturbance of the strata by faults is much greater than in any other British coal-field. The whole series is squeezed into a comparatively narrow trough, which throws the bottom of the basin to about 8000 feet below the surface. The coals are in some instances tilted up vertically, or even turned over, a kind of disturbance which is usually attended with considerable shattering of the strata. In one instance the upper series of measures have been shifted horizontally by an inclined or slide fault for a distance of about 200 feet above the: lower series. In spite of the difficulties caused by these disturb ances, coal seams of only a foot in thickness are regularly worked in Somersetshire, which is far below the limits considered to be profitable in other districts. The coal-bearing strata of Scotland 1 are confined to the 1 For the following account of the coal-fields of Scotland the writer is indebted to Mr J. Geikie, F.R.S.