724 J. EDWARD SPURR 
sarily proportionate to the amount of movement, since faults 
with slight displacement are often accompanied by zones show- 
ing profound trituration, while others of far greater movement 
show to a much less degree the effects of friction. The friction 
in each case seems to depend upon the angle of the chief stress 
to the sliding plane, rather than on the amount of movement 
along this plane. In heterogeneous rocks the amount of move- 
ment of a fault can ordinarily be estimated with more or less 
accuracy, the degree of closeness depending upon the nature of 
difference in the composition of the rock-mass. In such heter- 
ogeneous rocks the amount and direction of a fault movement 
must be judged by any available phenomenon or phenomena. 
By far the commonest variations in rock-masses which are con- 
stant enough to be reliable as data are sedimentary beds, and 
therefore the commonest means of measuring a fault movement 
is the separation of the two parts of an originally continuous 
stratum. On this account it is easy to fall into the error of 
considering faults simply as dislocations of strata. In careful 
geological work, however, such as mining work must necessarily 
be, it is important to cultivate a more correct conception, and to 
regard sedimentary beds as phenomena accidentally associated 
with faulting, whose dislocation must be associated with all other 
available criteria, each one as valuable as the other, to deter- 
mine the amount and direction of the total movement or dis- 
placement. Any fault, for example, in which the direction of 
movement is parallel with the plane of sedimentation will not 
cause any apparent displacement in a sedimentary bed ; and this 
may be the case in faults having any conceivable attitude, since 
the sedimentary beds themselves may be folded so as to stand 
in any conceivable attitude with reference to any fixed plane, 
such as the earth’s surface. 
When the direction of movement in a fault lies at a slight 
angle to the plane of sedimentation, the apparent displacement 
of a stratum resulting from this fault will be only a slight part 
of the actual fault movement; and it is only when the direction 
of movement is perpendicular to the plane of sedimentation that 
