252 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
the amount of movement, since faults with slight displacement are often 
accompanied by zones showing 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 movement 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 heterogeneous rocks the amount and direction of a 
fault movement must be judged by any available phenomenon or phe- 
nomena. By far the commonest variations m rock masses which are 
constant enough to be reliable as data are in 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 acci- 
dentally associated with faulting, whose dislocation must be associated 
with all other available criteria, each one as valuable as the other, to 
determine the amount and direction of the total movement or displacement. 
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, whatever may be the attitude of the fault plane in 
relation to the plane of the stratum; and this may be the case im faults 
having any conceivable attitude, since the sedimentary beds themselves 
may be folded so as to stand in every 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 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 
