The seams of coal not only vary in thickness, but they also differ in quality, and even the same seam alters in different and often in the same district. Sometimes a seam is nipped out and disappears from the series. In other places, it runs into and it may be said becomes part of other seams. In consequence of the gradual rise of the seams to the westward, and the uneven surface of the ground, particularly when undulated as in the valleys of rivulets and streams in the western districts, which valleys are still above the level of the sea, and the hills on each side of the Derwent and Tyne from 500 to 1,000 feet above it, one seam may come out much sooner than the other, the lower one at all times continuing further under the surface. In all the districts the whole of the seams of coal appear except near the western outcrop, that is to say, they are all found either in a good or bad condition, but at the western outcrop the Beaumont or Harvey, the Five-quarter, Stone coal, and Brockwell often run together and form only two seams before they appear at the outcrop. In the Black Boy district further east, as already remarked, it is questionable whether there are any more than two workable seams below the two seams that are at present being worked, and which, continuing westward, come into one at the outcrop. The sloping banks to the west of the coal district, and the rise of the coal in the same direction in many cases, cut the higher seams out a mile or two before the lower seams reach the surface, or cause them to become so much altered that they lose their distinctive characters, and it is difficult, if not impossible, to determine which seam they really are, and whether they possess any value or not.

Of course, the lower seams will continue longer under the surface westward than the upper ones, consequently, these upper seams cannot be measured as over the whole surface.

Those who have read Mr. Greenwell's work upon the western district will find that he makes allusion to this. The doubt which arises is, which name is right — whether the seam ought to called by one name or the other?

This is particularly observable on approaching the escarpment of the coal-field, where very few, and in many cases not more than a single seam appears. In the outcrop, two, and sometimes three, seams run together, but this arises from local circumstances, and does not afford any criterion by which to judge of the whole of the different coal seams, which vary in thickness, but correspond in number one with another, when properly traced through the various coal districts, except where good reasons can be given for their non-conformity. Sometimes the direction of a seam is suddenly altered; it is thrown upwards or downwards by hitches or slips of the strata. In the proved districts of the coal-field, these various circumstances and conditions are ascertained and recorded; and, from the information collected, the state of other districts where the coal has not been proved may be inferred. This is particularly observable on the map.

Household Coal. — The best quality of coal is considered to be that which has been found most suitable for household purposes, and termed on that account household coal. It has hitherto brought the highest price in the market, and is in universal demand. As regards price, it may only now be said to be excelled by a very limited portion of a certain seam below the Towneley seam at Blaydon and Stella, which is in high repute, and selling at high prices for gas purposes only. Indeed, it is only within the last half century that the seams which are now generally considered to yield the best household coal were opened out for working. Before this period there were certainly Wallsend, Heaton, Percy Main, and some other collieries on the Tyne that were working good coals, which have since been nearly worked off; but, even if that were not so, and these coals had been in the market, they would not have brought the price of what are now regarded as the best coal by 4s. or 5s. per chaldron. There was at that period a fine tender coal of good quality worked, which augmented the supply of household coals; at that time the household customers not being so particular either as to the size or quality. This coal, at that day, was very much used for smithies, and for the manufacture of gas, iron, and coke, and also for refining sugar. It is now worked extensively for coking and gas, as well as the other manufacturing purposes to which it was formerly applied, the demand having increased an hundredfold, and being likely to continue while cheap iron ore can be got.

Household coal is described generally as bright and cubical; it preserves its form in regular blocks after the small is taken from it. As an instance of this, it may be stated that one block was hewn at Black Boy Colliery, for the purpose of being sent to the Middlesbro' drops on the day of opening them, in 1831, and from thence to the Coal Exchange in London, weighing no less than three-and-a-half tons. This was the largest block ever received by shipment from the Northern Coal-field. This coal burns to a black cinder, which in turn is consumed, so that only a small portion of ashes remain. Its freedom from sulphuret of iron renders it admirably adapted for the purpose to which it is applied.

The best coals for household purposes, after being drawn to the pit mouth, are passed into waggons, in most cases, over a five-eighth inch screen, and the "duff" is that which falls through another screen, the latter leaving, of course, nothing but very small; only a very small portion of this coal is turned to refuse at the present time, for the dirt is sifted out of the duff; in some cases it is washed out by a machine invented for the purpose. The best portions of the duff are used for manufacturing purposes, in places where there is a demand for it. This was not the ease formerly, for nuts and duff were thrown aside and accumulated near the pit mouth, where storage ground was required to be provided. Hundreds of thousands of tons of these coals were annually destroyed as waste by burning.

This was, in fact, at that time the most economical mode the proprietors had of disposing of them, for they could not be sold at a price to cover rent and leading expenses, without the working charges, which were very considerable, so that all the weight came upon the large, and, besides, for many years there was a heavy duty chargeable on sea-borne coal, which of course tended to limit inferior sorts and to enhance the cost to the consumer. The abolition of the duty, and the repeal of other imports which crippled the coal trade, have been followed by an increased demand for coal of all descriptions, and the high mountains of black diamonds which once disfigured the surface of our fields, and invited the comments of strangers and travellers, have nearly disappeared. Nothing more clearly shows the policy of taking off a duty than the remission of the import of sea-borne coal, which, as in the case of glass, has led to a greatly increased consumption and its application to uses to which it was formerly unapplied.

The screening process, to which we have referred, was not resorted to with respect to coal used for coking or gas, iron factories, sugar refineries, or smithies, as the small in some districts was found to answer the purpose quite as well as the other on account of its fine bituminous nature. The steam coal, was, however, screened until lately, the practice having been abandoned since the increased demand arose, and the low prices at which these coals have been sold at the shipping places on the river Tyne.

Steam coal is distinguished by the rapidity with which it generates heat.

Its combustible elements readily unite and burn with great intensity, leaving a white ash, which, after a time, disappears through the fire-bars or the draft of the chimney. Instead of the dark cinders and bright clear fire produced by the best household coals, there is a large portion of dust and dirt which are by no means agreeable accessories to the parlour or drawing room.

This coal is principally used for the purpose of heating steam boilers, especially those of sea-going vessels. Its specific gravity is 1.26. A cubic yard of it weighs a ton.

Steam coal is now found in seams of considerable thickness, nearly from six to seven feet, of a different appearance from what is called splint or cannel in this district. It looks like the best household coal, but it burns like splint to a white ash. These three kinds are suitable for steam purposes, but more particularly that obtained from the thick seams north of Blyth, Cramlington, and others, as the splint and cannel coals are only found to be a portion of the seams in their beds; the thick seams can be worked to more advantage and in larger quantity than the thin ones to meet the demand.

Coking coal is characterized by the large quantity of carbon which enters into its composition, amounting, in a general way, to eighty per cent, in that obtained from Towneley, Marley Hill, Garesfield, Bowdon Close, Pease's West Pits, Hunwick, Byers Green, and Brancepeth Collieries, nineteen of the remainder consisting of gaseous matter, and only one residuum or ashes, making the best coke. The residuum may be even diminished by an improved oven, properly ventilated and managed, for which I am about taking a patent. It is more tender and finer than household coal, and rapidly crumbles or falls to pieces, and becomes dusty. It is laminated in structure, and open in what is called the facings or the fibres. It cakes or runs into a plastic mass on the application of heat, and yields a reddish white ash when burned. Coke may be considered as the fixed carbon of the coal.

It is manufactured upon an extensive scale in many parts of the western portion of the county of Durham, also on the Wear and the Tyne, in ovens built expressly for the purpose, either in the vicinity of the mines which yield coal of quality suitable for the purpose, or in localities where facilities exist for bringing the coal to it at a cheap rate, and disposing of the manufactured article to advantage.

There is a very marked distinction between cinders and coke, which is, of course, familiar to those in the trade. Coke if good, sinks in water, bad coke and cinders swim. The reason of this, observes Dr. Ure in his "Dictionary of Arts," is "The longer and higher the heat to which carbon from wood or coal is exposed, the more it contracts, and consequently the denser it becomes. The volatile combustible matter of the coal is employed in producing the requisite temperature for coking, and the oven is or ought to be so contrived as to retain the heat and a proper quantity of air a sufficient time to produce the necessary aggregation of the particles of the carbon, or in plain words, the condensation of the coke which is discharged out of the oven in one mass.

Splint coal is heavier in specific gravity than other coal. It is sometimes found very pure at the top of the Towneley, Beaumont or Harvey seam, which are all the same seam, but which are called the Main coal at Wylam, and Engine Main at Walbottle. At Towneley this top splint coal is very good, and it is also found very good at the bottom of the Brockwell seam in the three last named collieries, seldom running thicker than eight inches. It is equal in quality to the rest of the seams in which good splint cannel coal is found, but is generally worked in this seam along with the other portion, and is advantageously mixed with it in the process of manufacture of coke or gas, where there is always a great demand for it.

The Towneley splint coal, which is extremely brittle, burns away like wax, and leaves a considerable quantity of white powdery ashes. The Brockwell seam splint in the western collieries on the Tyne works more in blocks like cannel coal, and is also much in demand.

Cannel coal, when found in entire beds, as in Lancashire, is not of so brittle a nature as that called splint, and is worked more in blocks. It can be turned in the lathe, and will receive a high polish. The cannel coal is found under and forms the lower part of the Brockwell seam, the lowest of the workable coal seams; it varies in thickness from five to tea inches. When this coal seam lies deep on Tyneside, it is often thinner and more tender in its nature than what it is when it comes nearer to the surface. At Wylam, Towneley, Eltringham, Bywell, and Mickley, this coal is found in a good state. It is used for the same purpose as splint coal. It is very bituminous, and in making coke or gas it is mixed with the smallest of the best coal, which causes it to burn much quicker than it would do alone. The admixture is also beneficial in another respect, greatly accelerating and improving the process of manufacture. Occasionally in other coal districts, the splint or cannel coal is found in distinct beds, where it is generally of a much harder nature, as at Blaydon and Stella, and part of Grand Lease, where it is of another quality of coal, which is now being brought into operation, but which had not before been noticed as a workable seam in that district. It lies between the Towneley and the stone coal. It is very thin, and only worked of suitable quantity in small patches on the south side of the main dyke at Blaydon and Stella. It brings a good price in the market for gas purposes, being nearly equal in quality to the best gas coal in Scotland. On account of the thinness of the seam, viz., from two feet to two feet three inches, with a band in it, it is expensive to work, consequently where it is worked it has to be sold at a high price, and besides, there is only a very small quantity of it in this district. This seam, in a great extent of the district, is not of the same good quality, and consequently is not, nor ever may be workable with profit. The seam is so small, that particular kind of' workmen would have to be increased so as to obtain a greater quantity. It is not like the Scotch or Lancashire thick seams.

The coking coal seams, where they are tender, are very apt to have what are called partings, fibres, or facings, often at an inch or an inch and a half asunder. When these seams lie near the surface, these partings sometimes admit saturation with water, and after having been stagnant and drained off by working the coal from the dip towards the rise or surface, the facing, when broken up dry, presents a very beautiful appearance, being traced with lines resembling the colour of the feathers in a peacock's tail.