It must, however, be remarked, that, in the second coal formation of Werner, as noticed in the geological view of the situation of coal fields, that though the great beds or masses of greenstone overlying a coal-field do not rise into what may be denominated mounttins, they form, in this singular variety of the coal field, hills, seve ral hundred feet above the level of the sea, with rugged precipices, rudely columnar, in some instances nearly 200 feet high, tinder which coal is not only found occa sionally, but in amazing thickness, though not of great extent. This is therefore a rugged aspect, formed by a peculiar kind of rock, under which coal is occasionally found.
This, we apprehend, is all that can be said regarding the general aspect of a country, as to whether it contains the coal formation or not ; for when it is known that coals are found not only at great depths under the sea, 'eut at very considerable heights above it, even so high as to form with their concomitant strata considerable hills, we may, from these circumstances, infer, that coals are found under every variation of surface, such as the ocean's sandy shores, the recent and old alluvial covers, and also under every kind of varied hill and dale. But we must observe, that there are also vast districts of country with all these varieties of surface, and exactly similar in aspect, under which no coals are to be found. This spews how limited our conclusions are from the aspect of a country, whether it contains the coal forma tion or not.
It thus appears that no conclusions can be drawn, with any degree of certainty, front the aspect of the country, as to whether coals are to be found in it or not. But mineralogists, who have surveyed many countries, and who have had an opportunity of examining the peculiar character, outline, and forms of mountains and hills, arising from the nature or kind of rocks constituting their mass, conclude, from the aspect in general, very correctly, of what rocks they are formed ; but this dis crimination is not applicable to coal-fields, as the rocks which compose the various beds, alternating with the coal, are, in general, much softer than the mountain rocks, and form few or no precipices. Where the coal formation forms a group of hills, they have the appear ance of some of the transition class, with the exception of being softer in their contour and outlines ; but this does not afford a distinction of character sufficiently pre cise, as the long running ridges of hills of moderate ele vation, forming the hill and dale country which contains coals, present, like other ridges, a line of horizon nearly level, with few or no indentations, and no abrupt preci pices, excepting when greenstone or basalt occurs, either overlying the strata, or forming a dike in the coal-field, and rising above the other softer strata. This situation
of the greenstone is altogether different from that of the greenstone in the second coal formation of Werner. The long ridges containing coal, and forming what is termed a tame horizon, are very similar to the hills composed of chalk. It is to be remarked, that the line of bearing of the hills and dales is somewhat in the direction of the large rivers, or main trunks of rivers, which drain the district of water.
Since so very little can be depended upon, as to the conclusions to be drawn from the aspect of a country re o.
:, arding coals being found in it, the next step is to exam ine the surface, or alluvial cover, which, in most cases, hides the strata underneath from view. Were it not for this cover, the searching for coals and other minerals would be a very plain and simple process; and, as this cover is frequently many fathoms in thickness, and some times of very heterogeneous composition, the pitting or boring through it is often attended with considerable dif fiLulty.
If the district under survey for coal is composed of the bleak and sandy shores upon the margin of the ocean, or of the recent alluvial land at the mouth, and along the course of large rivers, whose waters move with a slow current, it very seldom happens that any symptoms of the kind of strata underneath can be discovered, even although these covers be examined to a considerable depth, as it is not common to find any fragments of the adjoining rocks mixed with them. We have seen these covers passed through to the depth of ninety feet with out finding even the smallest fragments of stone, nothing being found larger than grains of sand. As these sandy and earthy deposites, which cover the rocks upon the margin of the ocean, and by the sides of slow running rivers, are very flat, and rise inland with an impercepti ble aogle of elevation, it seldom happens that any disco very of the rocks can be made, either in the beds or sides of the rivers and brooks which flow through them ; therefore, in all such cases, recourse most be had to pit ting and boring, in order to ascertain the kind of racks underneath, and whether they belong to the coal forma tion or not. The plans commonly adopted for passing through the alluvial covers will be afterwards described, when treating of boring.