MEASUREMENT OF FAULTS. 253 
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; strize on the fault plane, showing the direction of move- 
ment; 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 actually to ascertain the movement of a fault. 
It is sometimes possible to find out the amount and direction of move- 
ment 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 general character, resulting from the usual conception of a fault 
as a dislocation of strata. The four terms generally employed are displace- 
ment, throw, heave, and offset. The words displacement and throw are used 
interchangeably, and commonly refer to the separation of beds by a fault 
as seen in a vertical section. Each of these terms is used by some to indi- 
cate 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 agreement, 
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 differentiate clearly the 
different functions of a fault movement, and I have adopted the following 
terms descriptive of the most important of these. These terms include 
nothing very novel in the way of nomenclature, but are intended simply 
to affix definite names to definite things. 
Dislocation and displacement are general terms, applicable to any part 
or the whole of a fault movement. Lach of the functions defined below, 
and to which specific names are given, may be called simply a dislocation 
or displacement. 
Total displacement is the distance which two points originally adjacent 
are separated by the fault movement. The line connecting these two 
