The usual explanation of normal faults is given in Fig. 31. The inclined fractures of the earth’s crust must often be converging, bounding, or enclosing large V-shaped blocks (_A_, _B_). If now, through any cause, as the folding of the strata, they are brought into a state of tension, so that the fractures are widened, the V-shaped masses, being unsupported, settle down, the fractures bounding them becoming normal faults, as is seen by tracing the bed _X_ through the dislocations. The single fracture below the block _A_ is inclined, and the stretching has been accomplished by slipping along it and faulting the bed _Z_ as well as _X_, the entire section to the right of this fracture being part of a much larger V-shaped block the right-hand boundary of which is not seen. But the united fracture below the block _B_ being vertical, any horizontal movement must widen it into a fissure, which is kept open by the great wedge above and may become the seat of a dike or mineral vein. The beds below the V may, in this case, escape dislocation, as is seen by tracing the bed _Z_ across the fissure. These pairs of converging normal faults are called _trough_ faults; and this is the only way in which we can conceive of important faults as terminating at moderate depths below the surface, and not affecting the entire thickness of the earth’s crust. Important reversed faults are believed to occur chiefly along the axes of overturned anticlines (Fig. 24) where the strata have been broken by the unequal strains, and those on the upper side shoved bodily over those on the lower or inverted side. An extensive displacement of the strata is sometimes accomplished by short slips along each of a series of parallel fractures, producing a _step_ fault. Faults cutting inclined or folded strata are divided into two classes, according as they are approximately parallel with the direction of the dip or of the strike. The first are known as _transverse_ or _dip_ faults, and the second as _longitudinal_ or _strike_ faults. The chief interest of either class consists in their effect upon the outcrops of the faulted strata, after erosion has removed the escarpment produced by the dislocation. [Illustration: Fig. 32.—Plan of a dip fault.]