In the gulf coast country, the presence of salt-domes, which are the producing structures, gives rise to small knobs or eleva tions that may be plainly seen. However, many such domes are not so indicated.
It is sometimes possible to plot the mapped horizon on a topographic map and the dips calculated by the "cutting-across contours" of certain beds. Coal seams or limestones are best used in this connection. This method is simply the determining of elevations of the chosen horizon by the topographic contour lines, and a map so obtained may give some clue to various structures.
In a country where workable coal seams exist, but owing to the fact that means of transportation are not handy such coals are not mined on a large scale, it is nevertheless common for many small country coal banks to be mined by the land-owners themselves for their own use. The drifts or openings for such mines are generally driven up the dip, so as to prevent the accumu lation or water in the coal bank; as drifts driven down dip will accumulate water, necessitating the use of pumps or siphons, as well as hauling the coal out of the mine against gravity.
The last resort is to map the structure carefully in the field with the use of the proper instruments. The general method is by determining the elevation of one or more key-horizons at many points. The success of such a work depends upon the exposures available, and proper identification: By a key horizon is meant any stratum that may be followed from place to place, or from outcrop to outcrop. If such outcrops are not continuous but are some distance apart, they may be identified by some characteristic peculiar to such a rock. Fossils offer the best and surest means of recognizing horizons, and therefore thin fossiliferous limestones make the best key-horizons. The columnar sections that are to be constantly made may show up some similarity by means of which a stratum may be identified. Lithology, or the character of the rocks, may be such as to be of value in this connection, however, only to a small extent, espe cially in the case of shales and sandstones, as they may change from one to the other within a short distance. Thus, a sandstone found in one hill may take the phase of a shale in the next, and if this is not recognized it may, mislead one in the interpretation of the columnar section. The topographic ter
races caused by resistant strata may be followed no other means are possible. Coals make very good key-horizons, but where many coal beds are found, care must be taken not to con fuse them. The shale partings in a coal bed may be such as to be of use in recognizing that particular seam. Prominent sand stone ledges may be followed and used as horizon markers, but must be handled carefully.
In following outcrops, it is possible to find some clue to them even when they are under •cover; thus many coal seams, lime stones and sandstones may have a spring issuing forth, and as the springs may be from a certain horizon, they may be used to some extent if carefully handled to find the outcrop of the forma tion. Again, many strata do not support vegetation and there fore the surface along them will be barren and cut up, giving the appearance of "bad-lands" on a small scale. A peculiar color of shale, if known to be above or below a key-horizon, may assist in finding the stratum looked for, and may indicate the points to be searched.
The methods of obtaining the elevations of the various key horizons are by means of the aneroid barometer, engineer's spirit level, transit stadia and plane table. The results obtained by the use of the aneroid are subject to the limitations of the instrument, and it is best adapted to a country that has many established bench marks, and where the outcrops are some dis tance apart; its advantage is the rapidity with which it may be completed, but folds so mapped are shown only in a general way, as the smaller variations of the contour lines cannot be obtained by such means.
Where good topographic maps are available so that the position of outcrops may be easily determined and indicated on the map, and where only one key-horizon is followed, the engineer's spirit level may be used to good advantage. Starting at a B.M. the elevation of the nearest outcrop is obtained, and the level is kept at that horizon and the rock followed from place to place along that contour, which will also assist in finding the outcrops as the elevation is being carried along.