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.

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 65°. 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 45°.

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 millstonegrit and shale. The millstone-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 without 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 gray 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.

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.

Section of the Coal strata at the Salines on Kenawha River, Ohio.

The strata beginning at the deepest are as follow:

1. Sandstone. Sixty feet thick.

2. Coal. Six feet thick. It is bituminous and is extensively worked for several hundred feet under the hills.

3. Bituminous Shale, and slaty shale, forty feet thick. This, both slate and shale, is filled with extinct species of plants. Every layer not more than the fourth, or the eighth of an inch thick, when separated, displays fresh

once ignited it will continue to burn until the bitumen are consumed. Several other miner in coal beds, in greater or less quantities, in mines, but those enumerated are the most co

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