As we proceed inland, the recent alluvial cover be comes thinner, and disappears at its line of junction with the old alluvial cover. which line is, in most cases, very distinct. This kind of cover forms frequently a hill and dale country, considerably elevated above the level of the sea. The rivers and brooks which intersect it often ex pose the rocks to view, so that it can be determined, from inspection, to what class they belong. Therefore, in such a country, the first step to be taken in the survey in search of coal, is to investigate the beds and sides of every river and brook, noting the kinds of rock which appear, and also their dip and line of bearing. Every piece of broken ground and quarry must be examined in the same manner.
Springs of water require also to be examined; and al though those which have an ochery deposit, are not to be supposed as affording unequivocal symptoms of coal, yet they are sometimes indicative of coal, or of ironstone ac companying coal. At the same time, it must be remark ed, that these ochery deposites are found in waters which drain through common clay ; and often very abundant from peat earth.
If, after heavy rains, which cause springs to flow co piously, small bits and grains of coal are found thrown out by the water, we have reason to conclude that coal is to be found at no great distance ; and the searching for it must be proceeded in accordingly.
lf, in the sides of rivers, brooks, or broken ground, a black sooty appearance is seen, this is frequently found to be the crop or outburst of a coal, and ought to be followed in the line of dip, by throwing an open trench. If it is the crop of a coal, the sooty appearance will be come more compact as the operation proceeds to the dip. Small shining angular pieces of coal will be found mix ed with it ; and at last coal, apparently solid, but still very friable, will succeed, resting upon the pavement of the coal. The trial must proceed until the roof is found firm, regular, and parallel with the pavement. It frequentlly happens that all the natural cracks and fissures of the coal near the crop are filled vvith clay, which gives the coal a very unpromising appearance ; yet this disappear s after proceeding a few fathoms more to the dip. When a coal is found regular as a stratum betwixt the pavement and roof, we conclude that the full thickness of that particular bed of coal is found, providing we see no appearance of any of the disloca tions or troubles before described, which must be kept in view when drawing conclusions from the appearance of coals, when thus found. A fter a bed cf coal is found
regularly formed, as before described, if its situation and thickness farther a dipping is required to be known ; or, if the strata lying either above or be low it are to be investigated in search of otnar beds of coal, these points must be ascertained by pitting and boring.
In the face of steep banks of earth, even when the ground is not broken, the crops of coals are frequently obsei wed by a black colour given to the soil; which appearance is also observed in lands which have been recently ploughed. When this appearance is seen in the face of a steep bank, the search for a bed of coal should proceed to the rise, or a little higher up the bank than where the black colour appears. But when this appearance is found in flat ground, a trench must be cut at right angles across it, so as to ascertain the line of dip ; and, when this is found, the investigation will go forward in the line of dip, as before directed.
If, by the sides of rivers or brooks, pieces of coal are found much rounded in the angles, it is to be inferred that the bed of coal from which the fragments have been detached is at a considerable distance from the spot where they arc found ; but if the angles arc sharp, it may be concluded that the bed of coal from which they conic is at no great distance.
In digging through the old alluvial cover which rests upon a coalfield, numerous fragments of coal, and of all the accompanying coal strata are found, as well as small rounded gravel of the mountain rocks, and large boulder stones. It is remarkable that the fragments of coal, and of the coal strata, are, in general, not worn by attrition, hut are sharp and angular ; which show, that the formation of this old alluvial cover was by water, in a very different state from that by which the recent alluvial cover has been formed, and is daily forming. Yet though these appearances give evidence that the district contains the coal formation, they do not point out in what direction the crops of the coals from which they have been detached are to be found. It is proper to re mat k, in this place, that though unequivocal symptoms of coal are found, and that it may be inferred that we arc within a coal formation district, yet we can conclude nothing satisfactory, as to the thickness of the beds of coal, until proper trials arc made.