The next point of consideration is the vertical section of the coal-fields before exemplified. Such sections are generally made in the line of dip and rise, on which account the section, Fig. 7. is in the direction of the dot ted line OP, Fig. 6. and the coal-fields, beds of coal, slips, counter dips, and crops, are marked with the same letters as in the horizontal section, which exemplifies the positions noticed in the horizontal section. These sections are to be considered only as diagrams, shelving within a very limited space the phenomena connected with coalfields, it being impossible to spew the several parts in equal proportions to one another, as the sections represent miles in extent, whereas coals are only a few feet in thickness, and the distance between them a few fathoms. In order, however, to shew the effect of a great slip upon a scale of proportion, Fig. 8. represents three coals a, b, c, rising southward 1 in 5 ; the coal b is 35 fathoms disLAit from the coal c, and the coal a is 10 fathoms distant from the coal b; the coals a and b crop out at the line of junction with the alluvial cover, but the coal c is cut off by the slip AB down to the south, forming the south coal-field, where the coals a, b, c, are found dislocated Lnd slipped down. The magnitude of this dislocation is measured by the taking the distance betwixt the pavements of any of the coals in the north and south fields, measured along the face of the slip, which in this instance is 50 fathoms. It will also be observed, from this figure, that though the distance be twixt the coals a and b is only 10 fathoms perpendicular, yet their distance, when they crop out, is 50 fathoms in a horizontal line ; and the distance betwixt the coals b and c being 15 fathoms, their distance, when they crop out, is 75 fathoms. It follows of course, that those coals which have a great clip, or make a great angle with the horizon, have the less difference betwixt their perpen dicular distance from each other, and their distance from the one crop to the other, where they meet the al luvial cover, whereas those coals which lie compara tively very flat, or at a small angle with the horizon, have a very great difference betwixt their perpendicular dis tance from each other, and their distance where they crop out or meet the alluvial cover, as may be seen by inspecting the figure. The proportion betwixt these is found by multiplying the perpendicular dist trice by what is termed the rise : thus, if the distance be twixt two coals is ten fathoms, and the dip 1 in 5, then 10 x 5 gives 50 fathoms for the distance in the ho rizontal section at the crop, or if the horizontal distance and dip are known, then the horizontal distance divided by the dip gives the perpendicular distance betwixt the two coals, viz 50 divided by 5 gives ten fathoms per pendicular distance as above.
Having thus endeavoured to show, that when coal fields are not viewed upon a limited, but an extended scale, that they form either circular, or more generally, long elliptical basons, or portions of this figure, though sometimes in an inverted position, it must from this cir cumstance appear evident, that every coal and its ac companying strata will crop out and meet the alluvial cover, excepting when they are cut off and prevented from doing so by slips, or where they form portions of an inverted bason, or take a counter dip, as may be seen by inspecting the preceding figures relative to the shape of coal-fields. In the foregoing examples, the coals and accompanying strata are considered as keeping nearly the same distance from one another in each coal-field, and that they keep nearly the same parallelism as to the angle of dip and rise, with the quality of the coal unal tered in the several coal-fields produced by the slips and dislocations of the strata. In many instances we find
this to be the case; but many changes are found to be pro duced, such as alterations in the angle of dip and ris.; of the strata before coming to slips or dislocations, a ge neral change in the angle of dip and rise in the coal-field formed beyond them, some of the coals varying in thick ness in the different coal-fields separatei by these slips, and sometimes the' thickness of the strata betwixt two beds of coal is doubled ; at other times the distance be twixt them is rendered so little in some instances, as to be no longer visible ; by which means, two seams of coal which were wrought as distinct beds of coal, are joined together, form one bed of coal, and wrought as such. Instances are also found of beds of coal compos ing a coal-field on one side of a slip disappearing on the other side ; but Ibis does not take place with the chief or main beds of coal. Differences, in point of quality, are occasionally found in the coals divided by slips and dislocations, though it is clearly evident that it is the same lormation of coal-field on each side.
Although coal-fields are of the form as before describ ed, it must be rernembered•that the extent of a single coal-field, in the line of bearing, is found to be several miles in length, suppose six miles ; that is, they are some times hound of this:extent, without any great dislocations, as to have the effect of forming new coal-fields ; and they are found in the line of dip and rise to extend above two miles. Other coal-fields are very small, not ex• tending to half a mile in the line of bearing ; yet, from the extent of the coal-basons, we must remark, that, in considering the shape of a small portion of a coal-field, and the operations connected with the working of it, the idea of the bason-shape is abandoned, and the coal field and beds of coal are considered as a regular lying in clined plane. This, upon a limited scale, is not far from the truth, considering the great extent of the basons; but this idea will evidently be found very erroneous the instant a coal-field is viewed upon an extended scale, and extensive operations pursued in it.
The next point to be described, is the methods, com monly pursued by mineralogists and miners in searching for coals, premising, however, that a clear idea has been formed of the facts before stated, as to the general natural phenomena connected with coal-fields. In mak ing a survey in search of coals, there are two distinct points of surrey to be attended to.
1st, The searching for coal in a district of country whine no coals are known to exist.
2d, The searching for coal in a district of country where the coal formation is known to be, or where coals are working at no great distance from it.
In searching for coals in a district of country where no coals are known to exist, the first point for considera tion is the general aspect and outline of the country tinder survey. Although, from taking this general view, we cannot determine whether it contains coal fields or not. yet, if the country is composed of hills or mountains, with steep acclivities, precipitous rocks, and sharp and rugged summits, with narrow %alleys betwixt the moun tains, we have no reason to expect that strata will be found there containing coal ; and excepting this very obvious and plain feature, as to aspect, we do not know any other upon which we can conclude with any degree of certainty, whether coals will be found in a district of country or not.