The boring tools are represented in Plate CCCXC. Fig. 1. in progressive numbers, viz.
The mode of conducting a series of bores in explor ing the strata for coal, is next to be explained.
Plate CCCXC. Fig. 2. represents a district of coun try, in which, after a regular survey, coal strata, and the appearances of coal, have been found, having an evident dip to the south, as represented in the Figure. In this case, a convenient spot should be chosen upon the north part of the district to be explored, in order that the dif ferent bores may be carried southwards in the line of dip ; the first bore is therefore put down at No. 1. sup pose to the depth of sixty yards In this depth it is probable many varieties arid alternations of the strata will be passed through, as exemplified in the sections of the strata ; but for the sake of perspicuity, only a few strata are shown in the figure, which strata are distinct ly inserted in a journal as to their quality and thickness, and specimens of each carefully kept. The bore No. 1. passes through the strata d, c, b, a, without finding any coal. And as in proving a district with a regular series of bores, it is more economical to do so by means of bores of a moderate depth, than with very deep ones, which are attended with great expense, provided there be no particular difficulty in passing through the alluvial cover, which might render a number of bores very ex pensive. Suppose that, in the first instance, bores of sixty yards deep are resolved upon, as at No. 1 before mentioned, and that the clip of the strata is 1 in 10, the consideration then is, at what distance from No. 1. bore southward, will a second bore at the same depth strike the first stratum d, found at No. I. ? The rule is, mul tiply the depth of the bore by the dip, viz. 60 by 10 gives 600 yards, which is the distance required. In this man ner the bores No. 2, 3, 4, and 5, are successively placed, with this particular exception, however, that the point of surface where the first bore is put down must be considered as the point of level to which the top of all the other bores relate. If therefore the top of No. 2. bore is 10 yards higher than the top of No. 1, that ad ditional depth must be added to the bore ere it can be expected to reach the stratum d. On the other hand, if the top of any succeeding bore is under the level of the top of No. 1, bore, suppose 20 yards, that bore will reach the upper stratum of the former bore at a depth less, by 20 yards, than the depth calculated from a horizontal line from the top of No. 1. bore. Ravines frequently cut the strata in various directions, as at A, so as to lay the strata open to view ; and if these are very deep, an advantage may be gained by boring there, and proving the strata to a considerable extent by a bore of moderate depth. In all instances where a series of bores are carried on, a correct and distinct profile of the surface ought to be made in the line of the bores. In No. 2. a coal is found near the surface, and another at the bottom of the bore, which last rests upon the first stra tum d, found in the bore No. 1. The bore No. 2. is continued a few feet farther, until it is decidedly ascer tained that it is has reached the stratum d. These two bores have thus proved the strata to the depth of 120 yards. No. 3. bore having been placed according to the rule before given, it passes through two coals near the surface, and after being put down nearly to its pro posed depth, it strikes the stratum h, which is the upper stratum found in No. 2. bore ; but as a coal was found
in No. 2. under the stratum b, the proof is rendered mote complete by continuing the bore through that coal. This bore, with the two former, proves the strata to the depth of 180 yards. No. 4. bore having been placed in the same manner as the former, a coal is found after passing through a few yards of the strata, and the bore is continued downwards until the two coals found near the surface in No. 3. have been passed through. This, with the three former bores, ascertains the strata under examination to the depth of 240 yards, so that to reach the lower stratum a of No. 1. the bore No. 4 would re quire to be put down to the said depth of 240 yards. The bore No. 5. being placed in the same manner as the former, a coal is found within a few yards of the sur face ; but, after reaching the depth at which the coal at the top of N. 4 bore should have been found, strata altogether different occur. In this case it is usual to go on with the bore ten or twenty yards farther than the intended depth of the bore, to ascertain the alternations of the strata. In some cases no coals of any value will be found, as in the present example, the cause of which is, that a slip or dislocation 13 of the strata has occurred, throwing up all the coals passed through in the former bores, and that of such a magnitude that the strata b, a, of the bore No. 1. are found immediately after perforat ing the slip ; and within a few yards of the surface, in place of being found at the depth of 300 yards, as would have been the case had the strata continued at the regu lar dip of I in 10, and no dislocation had taken place. Since dikes, slips, hitches, and troubles of every kind and degree, as formerly mentioned, occur in coalfields, examples of which are given in the preceding part of this treatise, it will easily be seen, by an inspection of these figures, what difficulties may occur in boring a coal-field which is much dislocated and troubled. Some coal-fields are so intersected with these as to bewilder the ideas even of the oldest and most experienced mi nets ; for instance, since slips are of every magnitude, as regarding the extent or distance the particular strata are separated in a vertical direction, it not unfrequently happens that a lower coal is thrown upon one side of a slip, directly opposite to an upper coal situated upon the other side of it ; so that if the two coals are of the same thickness, erroneous conclusions arc most inadvertently formed, and mistakes with the most cautious will take place. Very many instances might be given of these anomalies found in the practice of mining, but it would be endless to elucidate them by figures. In the exam ple given of a progressive set of bores, it is evident that the first coal likely to be found to the southward of No. 5. bore, and in the new coal-field formed by the up throw slip, will be that found in the bottom of No. 2. bore ; and if so, there is reason to conclude that all the other superincumbent coals will be found in succession southward, excepting some great dislocation or off cut of the strata takes place to interrupt their regularity. Caution must also be observed, in approaching primitive or secondary rocks, it being found, as before mentioned, that coals and their accompanying strata flatten in the line of dip towards these rocks, form a trough or bacon, from which they rise suddenly, and form edge or half edge strata upon the face of those rocks.