Lime stone, which abounds with organic remains, particularly if it is of some considerable thickness, is, in many cases, an index to a coal-field ; as it is much harder than the common strata of a coal-field, it crops frequently out to day, that is, it is distinctly seen at the surface. \Ve find also, that these thick beds of lime stone are commonly found in the lower series of the strata of the coal formation, for which reason, where ever a bad of this kind of limestone is found, the dip should be first ascertained, and then the coals searched for in the line of dip; for although coals, and their ac companying strata, exist to a great depth under these beds of limestones, it seldom happens that any coals of good quality, or workable thickness, are found there.
If a district of country is to be surveyed, in which are mountains. or elevated ground abounding with rocks, these require in the first place to be surveyed. In most instances there is no great difficulty in determining to what class of rocks the mountain formation belongs, as some part of them is laid open by the streams of water with which they generally abound. If it is found they are composed of primitive or secondary rocks, as before enumerated, it is certain that no coals will be found un der them ; th•fore the next point to be ascertained is, the dip of these rocks, because, in searching for a coal field, it will be necessary to follow the primitive rocks in the line of dip, as the others follow in overlying succes sion. If the coal formation exists in that district, it will be found by tracing forward in that direction, and if found at the foot of primitive or secondary mountains, the strata of the coal-field will he found lying in a conforming si tuation with the face of the mountains, and forming a very great angle with the horizon. 11 the coal formation exists at the foot of such mountains, it will be in most cases easily discovered in the ravines and hollows form ed by the mountain streams; and in many instances the coal and accompanying strata arc seen so clear and dis tinct, that the operation of mining and working the coal may be immediately commenced without farther investi gation ; but as the edge coals arc more difficult and ex pensive to work than the flat coals, it may be desirable to trace the coal-field farther onward. It has been found from experience, that when edge coals arc thus found resting upon primitive or secondary rocks, that they form part of a great boson, and that after dipping to a considerable depth, they begin to di fleet front their vcr ticular line and form a hollow, or, what is termtd by millers, a trough or boson, upon the opposite sick of which the edge coals are found forming flat coals, ris ing in an opposite direction, as represented in Plate CCCLX XXIX Fig. 7. where I is the transition rocks,
and a, b, the coals in the coal-field, A forming a trough or boson.
There are also instances of coal-fields where the edge coals have been wrought to a considerable depth with out any symptom of their deflecting, and adjoining them flat coals are found cropping or rising towards the edge coals, as represented Fig 9. We are inclined to think that, in this instance, the edge coals, a. b, c, pass under the flat coals, d, e, f, at a great depth, and deflecting from the vertical line form the flat coals of the boson, a, b, c, on the opposite side ; and that the great boson, formed by the edge coals thus deflecting, is filled up by a distinct set of flat coals and their accompanying strata, d, e, f. We arc of opinion that the very interesting coal field, south from Edinburgh, is of this kind of forma tion. All these varieties, now and formerly described, acquire to be kept in view. together with the various dislocations and troubles incident to coals. In prose cuting the search for coals beyond the edge coals, it ge nerally happens that a short distance from the mountain foot or rising ground, the alluvial cover prevents the rocks or strata being any more seen ; in this case, the whole district under survey must be minutely investi gated, in particular all broken ground, beds of rivers, quarries, and new formed ditches, where it is probable, it the alluvial cover is not very thick, some vestiges of the strata will be seen, but if no vestiges of the strata can be discovered, the next step is to proceed with pit ting and boring.
The foregoing rules are such as are at present adopt ed in surveying a district of country in which no coal is known• to exist.
The next case is the searching for coal in a district of country where the coal for motion is known to exist, or where coals arc working at no great distance from it.
If it is only known that the coal formation exists in the district, without any particular account of the beds of coal, the dip, line of hearing, or fat in of the ; the survey and investigations must he proceeded in ac cording to the rules laid down, connected with the par ticular circumstances of the surface of the country un der survey ; and if coal cannot be discovered by minute investigation, the search must proceed by means of pit ting and boring.