290 ELEMENTARY GEOLOGY 
world, great faults are developing; and every time the 
rocks slip for a distance of a few inches or feet, a 
disastrous earthquake shock extends over the neighbor- 
ing country. 
Terms used. In the fault, the plane of breakage is 
called the fault-plane (Fig. 171). Sometimes the fault- 
plane extends vertically into the earth, but usually it 
is inclined from the vertical, though it almost always 
dips underground at a high 
Se angle. The dip of the 
gee: fault-plane is called the 
hade, but the angle of hade 
is measured from the ver- 
tical, not from the hori- 
zontal, as is the dip of 
the rock stratum. Ex- 
Fig. 171. 
Diagram of a fault. AB, fault-plane; amining a faulted area, 
DE, throw; U, upthrow side; Dt, we find that a given layer 
downthrow side. : 
stands on the two sides of 
the fault-plane at different levels. The space by which 
the two ends are separated, or the vertical distance 
between them, is called the throw. The side that is 
uppermost is called the wpthrow side; that which is 
down, the downthrow. Whether one side went up or 
the other moved down, cannot usually be told, but 
almost always the movement is on one side only. 
Kinds of Faults. Where faults cross horizontal 
