THE MEASUREMENT OF FAULTS 7PM 
sary. The perpendicular separation thus has a certain relation to 
the lateral separation; for it constitutes a side of a right triangle, 
the hypotenuse of which is the lateral separation, except in the 
possible case where the perpendicular and lateral separations 
coincide. 
This mathematical relation makes it often possible to estimate 
the lateral separation from the perpendicular separation, and from 
the latter the total displacement. Of these three functions, the 
perpendicular separation is most easy of measurement, and its 
value may vary from zero to the full amount of lateral separation. 
The lateral separation is easier to ascertain than the total dis- 
placement, and its value may vary from zero to the total displace- 
ment. 
The measurements which have been defined have no constant 
direction, since they refer to fault movements which are capable 
of infinite variation. In general geological work, however, it is 
often only possible to measure fault movements along certain 
arbitrary planes. The most valuable of these planes, are the 
earth’s surface, which may be considered a horizontal plane, and 
vertical sections, into which available data are put, with the gaps in 
the chain of information often theoretically filled out. In such 
cases, where some dislocation is evident, but the information is 
so meager that it is not possible to know the fault so accurately 
as to estimate even approximately its total displacement or lat- 
eral or perpendicular separation, it is necessary to employ spe- 
cific terms to designate the known or estimated dislocations, 
although the relations of these dislocations to the total displace- 
ment may be unknown. For this purpose the terms offset, throw 
and vertical separation may be used. The terms ¢hrow and verti- 
cal separation are applied to the dislocations of a fault as seen in 
a vertical section; the term offsef to the dislocation as seen in 
a horizontal section, such as the earth’s surface may be con- 
sidered to be. 
A throw may be defined as the distance between the two 
parts of any body available as a criterion (such as a sedimentary 
bed), when these parts have been separated by a fault, the dis- 
