With regard to searching for coal in what is termed by Werner the second coal formation, composed of the newest flcetz trap rocks, it is attended with more diffi culties than are found in any other of the coal-fields. In this formation the coals arc found of uncommon thickness, in some instances 100 feet, the prevailing strata above the coals, are various alternating beds of soft slate clay having very little tenacity, with some times a few beds of sandstone ; and over these, next the surface, thick beds of greenstone ; which, where dislocations of the strata have taken place, form high precipitous cliffs, such as grcenstone precipices always assume. The coals found in these fields do not extend over a great district of country ; in some instances they do not extend to a mile in length, the coals lie irregularly, and in a short distance vary from lying moderately flat to very steep.
Coal-fields of this singular kind have been found more by accident than from regular surveys and mi nute searching, for this reason, that the masses of greenstone present such an unfavourable appearance, and so forbidding, that trials by boring would scarcely be recommended by a mineral surveyor, particularly as limestone is more frequently found under very thick beds of greenstone than coal, with this difference, how ever, that the strata accompanying limestone in such cases, are stronger and more compact. If, in such instances, no vestiges of coal can be found, where the alluvial cover can be cut through near the crop of the coal, the next step is to bore beyond the edge of the greenstone, and also to search for the softer strata ly ing under any of the greenstone cliffs, which when found, a bore may be put down to any requisite depth to ascertain the strata ; and if coals are found there, it is probable they will be found under the greenstone in the flat part of the country, which also must be ascer tained by boring. If coal is found in one place, its clip, line of bearing, and extent, require to be found accord ing to the process before described. These uncommon beds of coal are of various quality, some plias of them being hard, others sof', of a rich caking quality ; but by fat' the greatest part is of an inferior kind, having sul phur combined with it, and yields, when burnt, ashes of a dark reddish brown colour, which is one of the pe culiar characters of all sulphurous coals. Such coals are very liable to spontaneous ignition, after being open ed up and wrought for some time.
Glance coal, ,,hich is the blind coal of Scotland, and the stone coal of Wales, is sometimes found alternating with the other kinds of coal ; at other times, the com mon coals pass into this kind ; it is, however, more commonly found in Scotland, where greenstone abounds in the strata, and is very abundant in Ireland.
. No rules can be laid down for searching a district for this coal, beds of it have generally been found by acci dent.
For particular information regarding the different kinds of coal. (See the Article Coat.) Bovey coal, being wood, or great collections of trees found in the alluvial cover, semi-mineralized, and ly ing nearly horizontal, it cannot, in strict mineralogi cal langUage, be named coal, though some of it is so like coal, as to be termed brown coal. The only con siderable extent of this kind of coal which has been found, and continues to be wrought in Great Britain as fuel, is near Exeter in Devonshire. As its existence in any district of country cannot he traced by analogy, as coals can be, from a regular series of rocks, no rules can be given as to searching for it ; and it is appre hended that this kind of ligneous deposit is but spar ingly scattered over the globe. It is found in Europe, in the districts of Germany, Denmark, Prussia, Italy, Greenland, and the Faroe Islands ; and probably exists in other quarters of the globe. The drift-wood which has greatly accumulated, and continues to accumulate, along the sides of some of the great rivers of America, will, it is very probable, at no very distant period, be covered with alruvial soil, arid be converted into bovey coal. Front this view of its geographical position, it is more likely to be found in the recent than in the old al luvial cover ; and more probably not very distant from the line of junction betwixt them, where the rising grounds poll't out to the eye great districts of country, , which very evidently were occupied by water previous to the extensive districts of rich alluvial lands being formed by deposits from the rivers.
In surveying a district for coal, both lime and iron stones may be discovered. In some instances, lime stone forms the roof of coal, but in the extensive coal districts, the thick and valuable beds of limestone are found in the lower series of the strata, under all the main beds of coal. Limestone being very hard, and not easily decomposed, frequently crops or bassets out to day, hence more lime rocks have been discovered by surveying the aurfacc titan by any expenti‘e process of boring.