98 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
slight, and probably nearly dies out on the northwestern slope of the moun- 
tain. At this point the north-south faults of the Ontario system swerve to 
the northwest from the north and approach the Silver fault. 
Owing to the diminution in throw of the Ontario fault toward the 
south, at the southern edge of the Tourtelotte Park special map (Atlas Sheet 
XI) the downward throw to the east is only about 250 feet, as shown in 
Section F. Ifthe throw continues diminishing at this same rate the fault 
must entirely die out in a short distance. 
Butte fault system——The Butte fault is in some respects the most important 
and persistent of a numerous class of parallel faults which are well devel- 
oped in the area represented by this map. It has already been remarked 
that in some respects the area may be divided into a northern and a south- 
ern district, separated by the Butte fault. Keeping this arbitrary division 
in mind, it may be observed that the east-west faults of the Butte system 
are considerably more numerous and important in the northern part than 
in the southern. In the southern part there seems to be a comparative 
uniformity in the displacement, which is mostly a successive downthrust 
of the blocks between them to the south. In the northern half, however, 
there is no great uniformity in the displacement. The faults are more 
numerous and heavier than in the southern part, and the blocks melosed 
between them have been irregularly shifted up and down. The Butte 
fault, which has been taken as the dividing line between these two slightly 
differing districts, partakes of the peculiarities of the southern part, having 
a maximum upthrust to the north of about 400 feet. As is shown by the 
north-south sections, the throw of the Butte fault is diminished on the main 
ridge directly south of the Tourtelotte Park basin (see Section H, Atlas 
Sheet XV) to 50 feet; but this diminution is due to independent movement 
of neighboring blocks in this much agitated area, and the throw as meas- 
ured in Sections G and I is rather to be taken as the normal one. 
In this northern part of the Tourtelotte Park area the east-west faults 
are much more numerous and important than anywhere else in the whole 
district. Their intersection with the several north-south faults has produced 
many separate blocks. In the movement which has apparently gone on 
since the formation of these blocks each one seems often to have had an 
independent action, sliding up or down without any great dependence on 
the motion of the adjacent masses. Since in this district the strata do not 
