Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/16

6 iron become of a brick-red colour. Considerable quantities of fuel are found necessary at the kiln, and some parts of the rock are too apt to vitrify in the process, an accident to which the crystalline limestone of Sunderland is not liable.

Along the coast of Durham from Shields to Hartlepool, the uppermost bed frequently consists of a species of breccia, the cement of which is a marl-like substance consisting chiefly of magnesian carbonate of lime, and with this breccia wide chasms or interruptions in the cliff are filled. The next strata are thin and slaty, but lower down the stratification becomes less distinct. The colour of this rock is then light hair brown, the texture crystalline and cellular, from which latter cause it strongly resists the stroke of the hammer. The slaty variety occurs at Bolden hills, Marsden rocks, and numerous other places; its colour is white inclining to buff; dendritical marks may be found between the thin layers into which it easily breaks; and in Marsden lane and on the sea coast a flexible kind has lately been noticed by Mr. Nichol. In the neighbourhood of Sunderland the brown variety is generally quarried; it partakes of the nature of swinestone, and from containing some inflammable matter requires only a small quantity of coal to be reduced to lime. That worked at Denton, not far from the Tees, and analyzed by the Rev. J. Holme, is, I suspect, of this quality, for he mentions bitumen, as one of its constituents; whereas Sir H. Davy takes no notice of that substance in the rocks of Eldon and Aycliff.