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has modified the whole mass, we may affirm that no retreat of parts, no contraction in dimensions in passing to a solid slate can explain such phenomena as these. They appear to me only resolvable on the supposition, that crystalline, or polar forces acted on the whole mass simultaneously in given directions, and with adequate pow er."

COAL.

There is no subject, within the range of geology, of more importance than the natural history of coal, since the inhabitants of some countries are almost entirely dependant on its existence and quantity, for the comforts they enjoy during the cold season.

We have already described this mineral, and given some account of its manner of existence, when treating of rocks and formations generally. It remains here to give a more general and extended account of this important article, and to point out its indications, origin, associations, &c.

Form of Coal Beds. Nearly all coal formations are basin-shaped, or in that form, as before stated, which would arise from a deposition of strata in lakes, or ponds of various depths. Mr. Bakewell compares the shape to that of a muscle shell. "The position of coal strata,” says he, "in many coal fields may be represented by a series of fresh water muscle shells, decreasing in size, laid within each other, but separated by a thin paste of clay. If one side of the shell be raised, it will represent the general rise of the strata in that direction, and if the whole series be dislocated by partial cracks, raising one part a little, and depressing the other, to represent faults in the coal, it will give a better idea of the coal field than any description can convey."

"We are here to suppose that each shell represents a stratum of coal, and the partitions of clay, the earthy strata by which they are separated. The outer, or lower shell, represents the lowest bed of coal which may be many miles in extent. Now if a much larger shell be filled with sand, and the lower shell pressed into it, we may

consider the large shell to represent limestone, and the sand, grit stone; we shall have a model of the coal strata in many parts of England, and their situation over the metaliferous lime, with the beds of sandstone by which they are separated from it."-Geology, p. 117.

Searching for Coal. In most instances the inclination, or bending of coal strata, is such that the veins rise nearly to the surface, and would be visible, were they not covered by the soil or gravel. When this is the case, the removal of the soil by rivulets, or the accidental slide of a side hill, will uncover the strata, so that their dip and thickness can be determined. This is considered a very fortunate circumstance, because the boring for coal with. out some such indications that it exists in greater or less quantities, even in coal districts, is a very uncertain means of its discovery. Sometimes borings to great depths have been made in the immediate vicinity of large coal fields, without producing any greater conviction of the existence of the mineral, than the surface before indicated.

Fig. 28.

The cause of this will be seen 1 2 by fig. 28, where, suppose 1 is the coal vein, and 2 a stratum of sandstone, below which is limestone, and that the basin is filled to the surface with slate, clay, &c. Now on boring at 2, it is evident that nothing but sandstone and limestone would be found, though it might be within a few feet of the coal vein, while had the examination happened to have been made at 1, coal would have been found within a few feet of the surface.

Where a coal stratum comes to the surface, it is generally in a decomposed state, and so mixed with the earth, as to present no other appearance of coal than a darker color, when compared with the surrounding soil. Hence the real quality of the coal cannot be determined until it is taken from below the influence of the weather, and in general, its quality improves as it sinks deeper into the earth.

In examinations for coal, the dip and direction of the strata in the vicinity, when known, should be carefully observed; for if the dip is towards the estate on which the

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trial is to be made, it is probable that the coal may extend under it; but if the dip is in the contrary direction, the search ought not to be undertaken, since experience has shown that it would be useless.

Fig. 29.

d

The reason will be understood by fig. 29, where 1 2 3 4, are a series of coal strata dipping towards b. The unconformable strata c c, are sandstone, lying over the coal. Now suppose the coal vein 4, makes an outcrop at that point, on the estate of A., adjoining the estate of B, which lies towards b, then it is apparent that A would find only a point of the vein 4, on his estate, and that it would be useless to search in the direction of d, for coal, since the dip of 4 is sufficient to prove that none exists there, unless indeed another coal field should be found. Whereas, on the estate of B, though there might not exist an outcrop, still the dip of that on the estate of A, would make it highly probable, that B would find coal on his estate, though it might be too deep for working.

We have observed in another place, that coal has seldom, or never been found in hot climates. According to Mr. Bakewell, this mineral has rarely been discovered beyond the latitudes of 35° and 65o. It however exists in the province of Canton, in about the 30th degree of latitude. In this country, the great coal ranges appear to lie between the latitudes of 40° and 45o.

Indications of Coal. Although it is not certain that coal exists at any given place until it is actually found, still there are indications which might perhaps warrant the expense of a search, by boring in districts where coal has never been discovered. These indications are various, and to point them out requires much knowledge and experience on this subject.

In England, Mr. Farey states, that the coal districts incline to clay, and are generally of an inferior quality. When laid down to pastures small daisies and insignificant weeds, are more disposed to prevail than grass.

In these districts, water is generally procured at inconsiderable depths, and, when the faults are numerous, springs are common, and range in a line with the fault, for the reason already explained.-See Fault.

The face of the country where coal exists, is generally undulating, the hills being rounded and not mural or precipitous, and the valleys gently sloping and not deep as they are in granite formations. Sometimes, however, coal is found in hills more than a thousand feet above the general level of the country.

Strata which indicate Coal. In England and Wales, coal generally reposes on a series of beds called millstone-grit and shale. The mill-stone-grit is merely a coarse grained sandstone, consisting of quartzose particles of various sizes, agglutinated by an argillaceous cement. This differs from the sandstone that is found above and between the coal strata, chiefly in its greater induration. The shale is a dark colored slaty rock, which differs little from the slate-clay beds found among coal.

In England, secondary limestone is often associated with coal. This is called carboniferous limestone, because it is generally found in coal or carboniferous districts. Red marl is also a common attendant on coal formations, and indeed, is so common, that few coal shafts are sunk witout piercing through it. This is a kind of sandstone cemented with clay and colored with iron. Mr. Bakewell supposes that it has been formed by the decomposition or disintegration of trap, greenstone and granular quartz. Coal is also accompanied with thin strata of what the English call iron-stone. This is a dark brown or grey stone, of an earthy appearance, but of great specific gravity, being about three times the weight of an equal bulk of water. This stone is smelted for iron, and yields about thirty per cent. Another attendant on coal is that kind of limestone, in England, called lias. This name is said to be a corruption of the word, layers, probably because the strata of this rock are generally very regular and flat. The finer kinds of lias answer for lithographic stones. This rock alternates with lias clay, the whole formation sometimes being several hundred feet in thickness. This clay is highly impregnated with bitumen, and contains much sulphuret of iron, so that when

once ignited it will continue to burn until the sulphur and bitumen are consumed. Several other minerals are found in coal beds, in greater or less quantities, in the English mines, but those enumerated are the most common.

The annexed diagram, fig. 30, showing the different strata as they occur in a coal field near Mamsbury, will illustrate the manner in which most of these minerals are placed with respect to each other.

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The lowest stratum, or that on which the others are placed, as within a dish, and which also rises the highest, marked 1, is Old red sandstone. 2, Carboniferous limestone. 3, Millstone-grit. 4 4, Coal seams. 5, Coarse sandstone. 66, Red marl, or new red sandstone. 7, Lias. 8, Oolite.

In this country, although several of the strata occurring with European coal are wanting, still it is found to be associated with minerals of the same general characters. In Virginia, the strata which cover the coal are sandstone and clay-slate, the latter often exhibiting vegetable impressions. The coal mines of Ohio, are situated among strata of limestone, sandstone and clay-slate.

The following section and description is from Dr. Hildreth's valuable communication on the coal deposites of the valley of the Ohio, contained in Silliman's Journal for Nov. 1835.

Dr. Hildreth's paper not only relates to the Geology, but also to the Topography, and Geography of the Ohio Valley. It is illustrated with a map, several views, and many wood cuts delineating organic remains, and is among the most valuable and interesting communications on these subjects ever made to the American public.

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