Page:Popular Science Monthly Volume 8.djvu/178

166 carboniferous limestone had been uplifted to something near its present position. By measuring the whole length of the succession of limestone-strata that presents itself along the gorge of the Avon, and making the requisite allowance for their slope, the geologist has no difficulty in determining their thickness; and he can say with certainty that, if these successive beds of limestone were piled horizontally upon one another, in the same manner as when they were first formed, their total thickness would exceed 2,000 feet.

Further, you must think of these strata, not only as they present themselves at the surface, but as underlying all our coal-fields, and as probably extending very far beneath the newer strata to the southeast of the dividing band I have just spoken of. Thus, if you look again at the geological map, and notice how the great South Wales coalfield is surrounded by the blue band that indicates the carboniferous limestone, you must think of this limestone as really continuous over the whole of the included area, since it is met with at all points in which the coal-pits are sunk deep enough to reach it. And so in the midland counties, where the map indicates New Red Sandstone and later formations as the surface-strata, these, on being bored through, are found to have coal beneath them; and if we continue the boring downward through the coal-measures, we everywhere come to the limestone-base of this great and important carboniferous series. How far this series extends beneath the newer deposits which form the land of the southeastern portion of England, no geologist can at present say with certainty. If it really underlies them, it must be at an enormous depth, as the results of the Sub-Wealden boring have clearly proved.

Although we are accustomed to speak of the coal-basins of Northumberland, Durham, Yorkshire, Staffordshire, Gloucestershire, Somersetshire, and South Wales, as distinct and separate, it is important to bear in mind that they were probably continuous when the coal-measures were first formed, the "basins" not having then taken shape. This shape was given them by the great disturbance of the older crust of the earth which marked the close of the Palæozoic period, and which brought up the carboniferous limestone into the ridges that now constitute the borders of the basins.

It is this upheaval which has given us access to a vast storehouse of a material of the greatest value to man. Every Bristolian knows the use of this limestone, alike for building and for the making of roads; and the demand for it in the midland counties, to which the Severn affords an easy water-carriage, hastens the already too rapid demolition of his beautiful cliffs. When "burned," i. e., reduced by heat to the condition of "quicklime," it becomes—in virtue of its peculiar power of combining with water—the basis of all mortars and cements. It is as indispensable to the iron-smelter as the coal by which his furnaces are heated, since without its presence he could not reduce