The hills and mountain ranges that were thrown up at the close of the Carboniferous age were many times higher and broader then than they are to-day. Heat and cold and the storms of a thousand centuries, working by disintegration and erosion, have worn away their substance, the valleys and low lands are filled with it, and the rivers are always carrying it down to the sea. The peaks and the crests have been the portions of the elevations that have suffered most. It is often as though the tops of the anticlinal folds had been sliced off for the purpose of filling the valleys with them to the level of the decapitated hills. A great part of the coal measures have thus wasted away; in some portions of the anthracite district by far the greater part, including many valuable coal seams. When a fold or flexure of the earth’s crust has been decapitated in the manner mentioned, the exposed edge of any stratum of rock or coal is called its _outcrop_. The angle of inclination at which any stratum descends into the earth is called its _dip_. The direction of a horizontal line drawn along the face of a stratum of rock or coal is its _strike_. It is obvious that the strike must always be at right angles to the dip. That is, if the dip is downward toward the east or toward the west, the direction of the strike must be north and south. It is now apparent that if one begins at the outcrop of a coal seam and traces the course of the seam downward along the line of dip, his path will lie down the inclination for a longer or shorter distance, until the bottom of the synclinal valley is reached. This is known as the _basin_ or _swamp_. Here the seam may be comparatively level for a short distance; more often it has a mild vertical curve, and starts up the dip on the other side of the valley, which inclination may be followed till the outcrop is reached. If now the decapitated portion of the fold could be replaced in its natural position, we could trace the same seam up to and over the anticlinal axis and down upon the other side. As it is, we must cross on the surface from the outcrop to the place where the corresponding seam enters the earth. In the southern and eastern anthracite coal districts of Pennsylvania decapitation of folds to a point below the coal measures is general; the coal seams dip into the earth with a very sharp pitch, and the coal basins are often very deep and very narrow, striking into the earth almost like a wedge. In the northern or Wyoming district decapitation is not so general, the angle of inclination of strata is mild, and the basins are wide and comparatively shallow. In the bituminous districts, where the disturbance to the earth’s crust has been slight, the coal beds lie very nearly as they were formed, the dip seldom exceeding an angle of five degrees with the horizon. The exposures here are due generally to the erosive action of water. [Illustration: OLD OPENING INTO AN OUT-CROP OF THE BALTIMORE VEIN.]