THE MEASUREMENT OF FAULTS 725 
the separation of the parts of the faulted stratum is an accurate 
measurement of the movement. Theoretically speaking, the 
chances are infinitely against any such coincidence, and in actual 
practice it is rare that the movement may be even approximately 
estimated in this way. In mining geology it has been found 
that the most valuable criteria for measuring faults are, besides 
sedimentary beds, igneous bodies, such as dikes ; bodies of ore ; 
striz on the fault plane, showing the direction of movement ; 
and the composition of the fault breccia, which may show, in 
some degree, the amount of movement. By taking several of 
these criteria together it is often possible to actually ascertain 
the movement of a fault. 
It is sometimes possible to find out the amount and direction 
of movement immediately ; but more often it must be indirectly 
calculated, and to do this it is important to have clearly in mind 
the nature and value of some of the principal functions of a fault 
movement, and to have specific terms by which to designate 
them. The terms already in use are of a rather vague and gen- 
eral character, resulting from the usual conception of a fault as 
a dislocation of strata; the four generally employed are ds- 
placement, throw, heave, and offset. Vhe words displacement and 
throw are used interchangeably, and commonly refer to the 
separation of beds by a fault as seen ina vertical section. Each 
of these terms is used by some to indicate the distance along 
the fault plane between the broken ends of the bed as seen in the 
section, and sometimes the perpendicular distance between the 
parts of such beds, projected, if necessary. There is no agree- 
‘ment, however, which definitely assigns the terms to separate 
measurements, and, indeed, it is very common for a writer to use 
the terms interchangeably for one or the other function. Heave 
and offset are also used interchangeably, and are usually held to 
signify the perpendicular distance measured on a horizontal 
plane, such as the earth’s surface, between portions, projected, 
if necessary, of a bed separated by a fault. 
In mining work it is generally necessary to clearly differen- 
tiate the different functions of a fault movement, and I have 
