We shall now turn to a consideration of sudden earth movements and some of their effects, including earthquakes. Mention has already been made of the fact that, when pressures and strains are set up in the outer portion ("zone of fracture") of the earth's crust, the rocks yield mainly by breaking or fracturing because the rocks not being under a great load of overlying material are there brittle. The earth's crust has been fractured on small and large scales in many places during the long space of geologic time. Where one block of earth's crust has slipped or moved past another along a fracture we have what is called a "fault." Such displacements of rock masses vary in amount from less than an inch to some miles, and they constitute one of the most important features of the architecture of the outer portion of the earth. There are two types of faults fundamentally different as to cause. In one type (so-called "normal fault") the rocks suddenly yield to a force of tension; a fracture develops and the earth block on one side of the fracture or fault drops with reference to that on the other. In the other type (so-called "thrust faults") the rocks yield suddenly to a force of compression or lateral thrust, and one block of earth is pushed or thrust partly over another along the surface of fracture or fault. (See Plate 8.) Faults range in length up to hundreds of miles, those from one to twenty miles in length being very common. Where an earth block has been displaced thousands of feet along a fault surface, it is not to be understood that the whole displacement resulted from a single movement, but rather from a series of sudden movements separated by greater or less intervals of time. Each sudden movement along a fault surface produces a vibration of the earth near by. Many such sudden movements are known to have caused violent earthquakes. Displacements of twenty to fifty feet, as a result of single movements, are definitely known to have taken place in various regions during the last fifty years; and rarely, if ever, has any sudden displacement of as much as several hundred feet occurred. Cliffs and steep slopes very commonly result from faulting, but, because of the long lapse of time required for the repeated movements in the case of great faults, the cliffs or steep slopes begin to wear back and become more or less subdued long before the last of the movements take place. In regions where movements along great faults have long since ceased, the original steep slopes may be completely obliterated by erosion. [Illustration: Fig. 10.--Vertical sections through strata illustrating common kinds of faults: a, "normal faults" where one mass simply sinks below another; b, a "thrust fault" where one mass is shoved over another. (After U. S. Geological Survey.)]