Page:EB1911 - Volume 10.djvu/220

Rh exists as a prominent cliff-like face of rock, a “fault-scarp”; familiar instances occur in the Basin ranges of Utah, Nevada, &c., and many smaller examples have been observed in the areas affected by recent earthquakes in Japan, San Francisco and other places. But although there may be no sharp cliff, the effect of faulting upon topographic forms is abundantly evident wherever a harder series of strata has been brought in juxtaposition to softer rocks. By certain French writers, the upstanding side of a faulted piece of ground is said to have a regard, thus the faults of the Jura Mountains have a “regard français,” and in the same region it has been observed that in curved faults the convexity is directed the same way as the regard. Occasionally one or more parallel faults have let down an intervening strip of rock, thereby forming “fault valleys” or Graben (Grabensenken); the Great Rift Valley is a striking example. On the other hand, a large area of rock is sometimes lifted up, or surrounded by a system of faults, which have let down the encircling ground; such a fault-block is known also as a horst; a considerable area of Greenland stands up in this manner.

Faults have often an important influence upon water-supply by bringing impervious beds up against pervious ones or vice versa, thus forming underground dams or reservoirs, or allowing water to flow away that would otherwise be conserved. Springs often rise along the outcrop of a fault. In coal and metal mining it is evident from what has already been said that faults must act sometimes beneficially, sometimes the reverse. It is a common occurrence for fault-fissures and fault-rock to appear as valuable mineral lodes through the infilling or impregnation of the spaces and broken ground with mineral ores.

In certain regions which have been subjected to very great crustal disturbance a type of fault is found which possesses a very low hade—sometimes only a few degrees from the horizontal—and, like a reversed fault, hades beneath the upthrown mass; these are termed thrusts, overthrusts, or overthrust faults (Fr. recouvrements, failles de chevauchement, charriages; Ger. Überschiebungen, Übersprünge, Wechsel, Fallenverwerfungen). Thrusts should not be confused with reversed faults, which have a strong hade. Thrusts play a very important part in the N.W. highlands of Scotland, the Scandinavian highlands, the western Alps, the Appalachians, the Belgian coal region, &c. By the action of thrusts enormous masses of rock have been pushed almost horizontally over underlying rocks, in some cases for several miles. One of the largest of the Scandinavian thrust