In the western parts of the coal district, great abun dance of iron ore is found ; and tradition reports, that the Danes wrought it. It certainly appears to have been smelted at some remote period, from the immense heaps of iron slag which arc found in various places. They were certainly very regularly and extensively wrought in the middle of the 14th century. In the lead-mine dis trict, a few miles to the north of Stanhope, there is a large quarry, from which grey or freestone millstones, of a very superior quality, are procured. The bed of stone is 21 feet thick ; but not more than half of this con sists of real millstone. Above it lies a seam of liner freestone. About 40 millstones are sold on an average in the course of the year. What are generally called Newcastle grindstones are got on Gatesheadfell in this county. They have been wrought for a great number of years. Some are cut as large as 76 inches in diameter, and 14 or 15 inches thick. A cubic foot of them weighs 1 cwt. 1 qr. 14 lbs. th•-nsand chaldrons, each chat we - rat; are made annually. In times of p - deo and f, r tht in is front Ilollat d I 1 n e r t. I n rs kinds of freestone found in eh Wei in pit is t f tin that four d at 11 eworth ...,t of notice. It is of all open, potnus na tt u r, atql•t.11 s the extreme heat of the glasshouse fur ince Y blt'l u th–n any other stone or bricks. In the wes tern parts of Durham, grt y slate for roofing is abundant. aa'hat is tailed sitter sand, and is used in the manufac mre of the finer kinds of glass, has been discot creel at Seal ann.
llot next to coal rind lead, limestone is the most abun dant in Durham. The positions and limits of the differ ent hint moue districts have already been pointed out. In Weardale, near Froste rly, a limestone is found that takes a fine polish, and the colours arc so beautifully varie gated, that it is frequently used as marble for chimney pieces; at Pallison also, several strata of limestone have been lately discovered, which approach still nearer the quality of mai ble: they arc clouded with all the s arie tics of brown, hum a dark brown to a cream colour. This limestone takes a very fine polish, stands any wea ther, and will net fly with heat : there arc four strata of this fine limestone, each three or four inches thick ; they lie near the bottom of the quarry ; above them is lime of the common quality. For the purposes of agriculture, it has been found that the lime from the quarries to the eastward of the coal district, differ in their effects from the lime brought from the quarries to the westward of this district : the former, when laid on the land in heaps, and suffered to lie some time before it is spread, renders the ground under it unproductive; and hence it is called, i• burning lime ;" whereas the other lime produces no such effect, and is called " mild lime." These limes have been carefully analyzed by Sir liumphry Davy and other chemists ; and the result is, that the limestones to tire cast of the coal district, that is, the hot or bunting lime, contain a large portion of magnesia; and those to the west of the coal districts, called mild lime, arc per fectly free from magnesia. The purest of the mild lime
stone contains 96 of carbonate of lime, and only 4 of re siduum ; and the worst of the burning lime contains only 4l of carbonate of lime, 42 of carbonate of magnesia, and 14 of residuum. The had effects on the land were uniformly found to be in proportion to tire quantity of magnesia which each contained. According to Mr Mar shall, the limestone of Sunderland contains 97 of carbo nate of lime. This limestone is of a lightish grey colour; the rock near Sunderland, out of which it is wrought, is upwards of 50 feet high; it is covered with 10 feet of pale clay ; there are no regular seams; but it is com posed of huge blocks ; the interstices arc filled with a species of marl.
About thirty years ago, a salt spring was discovered in an engine pit, constructed for drawing water out of the coal mines ne::r Binley. As it mixed with the fresh water in the same pit, it would probably have remained unnoticed, but for an accident which happened to the boiler, the bottom of which suddenly (trout out. On examining it, it was discovered to be corroded, and en crusted with a large quantity of strong salt. On this the workings were examined, and the springs were found to be confined to a stone drift, that had been driven 200 yards in a north-cast direction into the mine. No trace of it is detected any where else, though the pit, and every other near it, have been excavated, both above and below it many fathoms. The depth of the spring is about 150 yards. it is conveyed to the bottom of the pit, whence it is raised by the colliery steam-engine. The spring has never been known to vary, and is as strung now as when first discovered. About 1100 tons of salt arc made annually. It produces about 20,000 gallons per day, which is four times stronger than any sea water. At Ilutteiby, near Durham, in the bed of the river Wear, is a spaw, arising from a spring of water, strongly im pregnated with salt and sulphur. From the circumstance of its being mixed with the fresh water of the river, it is difficult to ascertain what quantity of salt it holds in solution ; hut, on several trials, it has produced double the quantity obtained from sea-water. It is much resort ed to fur its medicinal qualities. Doresdalc spaw is sul phurcous : it was discovered, in 1789, in searching for coal. The spring burst forth, with a strong sulphurous smell, at the depth of 24 yards, after boring through red freestone and whinstone. To the south of I lartlepool is a chalybeate spring, covered every tide by the sea. It is impregnated with a small degree of sulphur;—a gallon yields 12U grains of sediment, in which there appears to be also a considerable portion of lime. In the bed of the river Tees, near Barnard Castle, a sulphurcous water springs from the crevices of the rocks. It is of the same quality as Harwich water, used for the same diseases, but not so strong.