If there is an outcrop of coal on the tract to be mined, and the dip of the seam is more than twenty degrees, it is usually advisable to enter the mine by means of a slope. This is a passage which, beginning at the outcrop, follows the coal seam down until the necessary depth is reached. It is driven in the coal. The distinction between the drift and the slope is that the drift is driven from the surface on the strike of the seam while the slope is driven on its dip. Where the coal seam comes within a moderate distance of the surface, as at an anticlinal ridge, a slope may be driven through the rock until the coal is reached at the axis, and from that point follow the seam down. Sometimes a shaft is sunk to the top of an anticlinal ridge, and from its foot two slopes are driven, one down each side of the roll, in opposite directions. If the seam is very irregular, or if it is much broken by faults, there may be a great deal of rock cutting to be done in order to preserve the uniformity of grade necessary for the slope. The cost may, indeed, in this case, amount to more than would have been sufficient to sink a shaft to the same depth, although, as a rule, the entrance by slope should cost only about one fourth of that by shaft. [Illustration: CROSS SECTION OF SLOPE WITH DOUBLE TRACK.] The same methods are employed in sinking a slope as are used in driving a drift, except that generally the timbering need not be so heavy. The minimum height of the slope is about 6½ feet, the width at the top, or collar, about 8 feet, and the width at the bottom, or spread, about 12 feet. If a double track is desired the spread should be 18 feet and the collar 14 feet. In the Wyoming region, where the dip is usually less than twenty degrees, with infrequent outcrops, the slope is not in general use; but in the Southern coal field, where the dip varies from twenty degrees to the vertical, the slope is the most common method of entering a mine. There the opening is driven down for a distance of 300 feet, at which point gangways are started out to right and left, along the strike, and chambers driven from them back toward the surface. This is called the first lift. The slope is then continued downward for another distance of 300 feet, new gangways and chambers are laid off, and this is called the second lift. This process is continued until the synclinal basin is reached. Where the dip of the slope is less than thirty degrees the coal is brought to the surface in the car into which it was first loaded in the mine. At a greater angle than this the ordinary mine car is superseded by a car or carriage especially adapted to carrying coal up a steep incline.