728 J. EDWARD SPURR 
tance being measured along the fault plane as shown in a verti- 
cal section. 
The vertical separation is the perpendicular distance between 
the intersection of the two parts of any faulted body available 
as a criterion (such as a sedimentary bed), with the plane of a 
vertical section, the lines of intersection being projected if neces- 
sary for the purpose of measurement. In perpendicular faults 
the vertical separation is identical with the throw; in all others 
it is less than the throw, but sustains a certain relationship to it, 
being one side ofa right triangle of which the throw is the hypote- 
nuse. Thus the vertical separation may vary from zero to the full 
amount of the throw. The throw is always a part of the total 
displacement, although with no definite relationship to it, and 
varies from zero to the full total displacement. 
The term offset may be used to designate the perpendicular 
distance between the intersections of corresponding: plane in 
the two parts of any faulted body available as a criterion, such 
as a sedimentary bed, with a horizontal planes such as the 
earth’s surface may be considered to be; the planes being pro- 
jected for the purpose of measuring, if mecessary. like the 
throw, the heave or offset is a part of the total displacement, but 
has no definite relationship to it. 
Tosum up, there are six terms proposed to designate the differ- 
ent parts of a fault movement, each term applying to a measure- 
ment which varies in accuracy and proximity to the total dis- 
placement in proportion to the available amount of information. 
For general outline work where accurate data are not obtainable, 
the terms “row and vertical separation, referring to the measure- 
ments of a fault at its intersection with a vertical plane, and the 
term offsef, indicating a measurement of a fault at its intersection 
with a horizontal plane, are adopted. The throw and offset are 
parts of the actual fault movement, but of unknown value, while 
the vertical displacement sustains a certain relationship to the 
throw. Where more complete data are obtainable, the terms 
total displacement, lateral separation, and perpendicular separation are 
adopted. The perpendicular separation sustains a certain rela- 
