In a paper devoted to the coal-field, it only seems necessary briefly to notice the strata more immediately in connection with it.

The coal measures throughout the whole of this district rest upon the mountain limestone and millstone grit, which crop out beyond the coalfield westward, and form the chief depository of lead ores, which are extensively worked at Allenheads, on the estates of W. B. Beaumont, Esq., M.P., and at Weardale and other places on the edge of and sometimes within the coal measures.

The strata of the coal measures, or those which form the chief depository of this valuable mineral, are, of course, familiar to the members of this Institute, from sinkings which have been made through them. They consist of shales, grits, and clays, and the great abundance of impressions of plants found in them, particularly those of luxurious growth, such as the fern, &c., indicate and sufficiently attest their vegetable origin.

Sections of the strata, from the highest to the lowest workable coal seam, in sinkings occurring at different parts of the coal-field, will be subsequently given.

Immediately overlying the coal measures, where they lie at a great depth near the sea, is the lower new red sandstone, which is very pure, and unmixed with shale; and above this, over a considerable portion of the county of Durham, we find the magnesian limestone, which in some parts is overlaid by the upper new red sandstone.

One remarkable effect produced by the dykes displacing the strata upwards or downwards, is the throwing in of patches of stratification in places where they would not he expected to make their appearance. Near to Whitley, where there is a small patch of limestone, the lower new red sandstone is found cropping out beneath it. At Killingworth, in consequence of the downthrow caused by the great Ninety-fathom Dyke, which runs from the sea by Shire Moor through Killingworth, where it must be a very great downcast to the north, a patch of the same formation is thrown in, being the only instance of the kind that is known in the county of Northumberland.

In the county of Durham, at Hessleton Dean, a similar case occurs with respect to the new red sandstone, which is thrown in with the displacement of strata by the Butterknowle Dyke, which throws the strata forty fathoms downwards. This is the only known instance that occurs of the new red sandstone being thrown in in that manner.

As the coal measures dip towards the sea in a south-easterly direction from the general line of outcrop, which varies in distance, proceeding from the sea westward, it is in the latter direction that we may expect to find the several escarpments of overlying strata. If, on proceeding westward, we find the lowest workable coal seam, the Brockwell, at the surface, proceeding south-eastward towards the sea, the several members of the series of super-strata ought to make their appearance successively. They undoubtedly do so with certain modifications, arising out of a variety of causes.

With respect to the coal series itself, when the seams lie at a great depth from the surface, they may he traced with considerable accuracy from one district to another. But on approaching the surface westward, these seams are found nearer to each other. Sometimes two unite and form one seam, or the occurrence of a slip may throw a seam out altogether, when all further trace of it is lost. The slip may occur in the opposite direction, and instead of throwing the seams out at the surface, throw them down perhaps several fathoms. Thus we find certain collieries, at no great distance from each other, differing very considerably in the number and quality of workable seams. From these and other circumstances, therefore, a section of the coal measures, say at Monkwearmouth Pit, will not find its counterpart on the surface at the coal measure escarpment westward. Some of the seams have run one into another; others have been nipped out or thrown out in the distance between the two places, particularly if that distance is very considerable. The debris of the surface, however, affords a certain indication of the strata below, and hence the line of the several escarpments can be accurately defined and delineated, as on the map produced.

The debris of lower new red sandstone, intermixed with other strata of the coal measures, fills up the interval between the coal outcrop and that of the magnesian limestone, the existence of which upon the strata found beneath it, has given rise, at various periods, to much discussion and inquiry with respect to the character and quality of the coal sub-strata.

The escarpment of the magnesian limestone, as traced upon the map, runs in an irregular line. A small patch of it lies north of the main dyke at Whitley, in Northumberland, at Tynemouth Priory, and it strikes in again at South Shields after crossing the Tyne, and continues on across the Wear at Pensher Hill, Newbottle, Eppleton, Hetton, Moorsley, Elemore Hall, Pittington, Sherburn Hall, Coxhoe, Haswell, Shotton, Ferry Hill, Coundon, Thickley, and Middridge, into Cleveland in Yorkshire.

The outcrop of the lower red sandstone is shown with a thin intermediate stratum of shale in some places immediately beneath it, particularly where it is deep. The sandstone appears on the surface in many places as fine soft sand. As this sand, or lower red sandstone, dips under the limestone it l)ecomes more compact, and at a great depth is found hard with gullets or cracks in it filled with water, which comes off into the shaft, when sinking, in enormous quantities, bringing with it particles of sharp sand, so that the leather buckets used for pumping (a practice I would not adopt, as those buckets, which must be worked with great rapidity and power to overcome the influx, are made of leather,) do not last many hours when the water is at a heavy pressure, in consequence of its great depth. It may be remarked that in one case, that at South Hetton Dalton Colliery, the cost of the leather for the buckets, was at the rate of £100 per week for a year or two, while little or no progress whatever was made in sinking until they got through the bed of sand, which difficult part was only a few feet in thickness. A similar occurrence took place in the first pit sunk at Haswell, where the sand was softer and much thicker than at South Hetton Dalton Pit, which is further to the dip, and the proprietors in consequence were obliged to abandon it before they got through the sand, after spending upwards of £60,000. They afterwards sunk a short distance westward, to the rise, where they had to sink through the same strata, and to their agreeable surprise, the sand being nipped out, there was little or no water found.

With respect to the magnesian limestone in the county of Durham, and the differences of opinion prevailing with regard to the quality and character of the coal sub-stratification, a few remarks here may not be out of place.

The author of a popular work entitled "Our Coal and Coal Pits," extols the sagacity of Dr. Wm. Smith, the father, as he may be termed, of English geology, and inferentially depreciates that of some of the late colliery viewers on the Tyne, who, it is reported, entertained an opinion regarding the quality of the coal underlying the limestone formation in the county of Durham, and the expense of working it, which was unsupported by theory and not justified by experience.

Now, so far from this being the case, it turns out that experience is in full accordance with what was predicted by the late Mr. John Buddle, and the viewers referred to, so far as experience has gone and new facts have been elicited. A very great portion of the coal under the limestone has not yet been proved. That which has been proved has not been of sufficient extent, or found to be of sufficient thickness and good quality to warrant any one in making any positive contradiction. The Hutton seam under Moorsley estate was worked a short distance under the limestone and found to be good, as was known to the late Mr. Buddle, as well as myself, at the time he is said to have made the remark that his opinion did not go so far as to lead him to believe that any great extent of the good Hutton seam would be found under the magnesian limestone.

Since then, the experience of thirty years has thrown additional light upon the subject, and I bold that his opinion was consistent with truth, and it has been confirmed to a considerable extent.

A great portion of the coal under the limestone, as already stated, has not yet been proved, particularly where the limestone is overlapped by the new red sandstone. In that portion which has been proved, the Hutton seam, which is considered the most valuable coal, is of unequal thickness. A great portion of it is so thin that it is unworkable with profit; and, where this thinness occurs, there is only one seam above it that can be considered of workable good quality, namely, the Five-quarter, which is only a thin seam compared with the good Hut ton seam; and although of good hard quality where of sufficient thickness to be workable, in many districts, it is not in at all, and in some cases where it is in, it is so thin and had in quality, as at Croxdale, Whitwell, Byers Green, &c., that it is not workable. Therefore, even the Five-quarter cannot be taken into account as a regular workable seam throughout the proved coal district under the limestone. When the Hutton seam is thin, and the Five-quarter above it is not workable, recourse has been had, in some cases, to the seams below, which require deep sinkings, which when reached are also found to be comparatively thin and tender. This coal is suitable for coking and gas, but does not bring the price of the best Hutton seam and Five-quarter or Main coal at Tees Wallsend by one-third.

The remaining portion of the Hutton seam under the limestone, which may be considered good, lies south of the Hett Dyke from Easington to Seaham, next the sea, and from Seaham or Ryhope, or a little north of it, to Pensher on the Wear, and thence to Durham, and by way of Shincliffe and Sherburn to the Hett Dyke, near Thornley.

The area thus circumscribed, it will be seen by reference to the map, is only of very small extent; and little more than one-half of it, namely, that portion next the sea, is overlaid by limestone.

In that portion under the limestone where the Hutton seam has been proved to be in good state of perfection, a few deep pits have been sunk down to it 'through the limestone, and a great many other pits have been sunk on the borders of the limestone, working underneath it as well as to the rise in the opposite direction, and these pits have already worked a large portion of good coal from under the limestone, and those with others working to the rise, have worked out nearly the whole of the Hutton seam up towards Durham.

Therefore, the pits on the border of the limestone already referred to, and others sunk to the rise in that portion of the good Hutton seam district which is not under the limestone, have exhausted, or will very shortly exhaust, the whole of the workable seams that can be got in that division, and which are known as the Five-quarter, Main coal, Low Main, and Hutton.

The seams below the Hutton in this district are yet unexplored. At Haswell, the Beaumont or Harvey, the next workable seam to the Hutton, has been bored to at the customary depth. As to the seams below the Beaumont, their thickness has not yet been ascertained. Even the number of workable seams below the Hutton in this district is doubtful. Indeed it is questionable whether there be more than two; for, although in the Tyne there are four workable seams proved below the Hutton, averaging upwards of a yard in thickness, these lie from ten to twenty miles beyond the outcrop of limestone at Tynemouth, whereas, at Black Boy, in the Tees Auckland district, where the limestone runs off, about twenty miles from the sea at Hartlepool, there is no proof that there are four workable seams below the Hutton, and it is doubtful whether there are more than two there, and although they lie at a great depth, they rise out to the surface within one mile southward, and a few miles westward, by way of West Auckland and Etherley, as may be seen upon the map.

Upon the whole, that portion of the coal-field in Durham and Northumberland, which is covered by the magnesian limestone, amounts only to about one-sixth of the entire area, and in only one-half of that can the Hutton seam be considered good; in the other half it is not workable with profit. The same observation applies to several of the other seams, particularly the Main coal and Low Main, which have been sunk to in various places.

Having disposed of the collateral question of coal under the limestone, it only remains to state that the new red sandstone overlaps the magnesian limestone from North Hartlepool to near Middridge, where the Tees Wallsend seams crop out.

Its northernmost point is at the Hett Dyke, between Hutton Henry and Wingate Grange Colliery. Southward it continues thickening to the Tees, till it is overlapped towards Redcar by the alum shale.

A little to the south-west of Redcar are situated the now extensively worked iron ore districts of Cleveland.