The anthracite coal beds were in the regions of greatest disturbance, and, together with the rock strata above and below them, assumed new positions, which were inclined at all angles to their old ones of horizontality. More than this, the heat and pressure of that period exerted upon these beds of coal, which up to this time had been bituminous in character, resulted in the expulsion of so large a portion of the volatile matter still remaining in them as to change their character from bituminous to anthracite. Although the strata, in the positions to which they have been forced, are at times broken and abrupt, yet as a rule they rise and fall in wave-like folds or ridges. These ridges are called _anticlinals_, because the strata slope in opposite directions from a common plane. The valleys between the ridges are called _synclinals_, because the strata slope from opposite directions toward a common plane. One result of this great force of compression exerted on the earth’s crust was to make rents in it across the lines of strata. These rents are called _fissures_. Sometimes the faces of a fissure are parallel and sometimes they inclose a wedge-shaped cavity. This cavity, whatever its shape, is usually filled either with igneous rock that has come up from the molten mass below, or with surface drift or broken rock fragments that have been deposited there from above. Where there is displacement as well as fracture, that is when the strata on one side of a fissure have been pushed up or have fallen below the corresponding strata on the other side, we have what is known as a _fault_. Sometimes the displacement seems to have been accomplished with little disturbance to the sides of the fissure; at other times we find, along the line of fracture, evidences of great destruction caused by the pushing up of strata in this way. A fault may reach a comparatively short distance, or it may traverse a country for miles. The vertical displacement may be only a few inches, or it may amount to hundreds or thousands of feet. In the bituminous coal regions, where the strata lie comparatively undisturbed, faults are but little known. In the anthracite districts they are common, but not great. [Illustration: VERTICAL SECTION THROUGH SOUTHERN COAL FIELD.] [Illustration: VERTICAL SECTION THROUGH NORTHERN COAL FIELD.]