250. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Black Rock seems to indicate that in this portion of the St. George 
block such an extra uplift took place. 
The relation between the upheaval of the Colorado plateaus and the 
cutting of the Colorado canyon has been explained time and again. 
Previous to the faulting the river flowed close to the level of the land 
surface as a whole. Its course at that time may have been in general 
consequent, as Davis has pointed out (4, pp. 158-167), but as yet we 
cannot fully restore the topography and structure of the entire region 
previous to the last faulting, and so cannot be certain on this point. 
When the plateaus were uplifted the rate of motion was so slow that the 
Colorado was able to intrench itself without appreciable change in its 
location, although the slight relief of the Mohave peneplain would not 
have been sufficient to prevent the river from shifting northward toward 
the Virgin had the uplift been very rapid. As it was, the Colorado cut 
for itself a deep trench along a course which, however consequent it may 
have been in the inter-fault cycle, seems to be strictly antecedent so far 
as the present canyon cycle is concerned. 
Minor Faults. 
In addition to the great faults along the border between the Plateau 
and Basin Range provinces, there are two minor faults in the vicinity of 
Toquerville. In both of these, as in all the known displacements of the 
region, the uplifted portion lies on the eastern side. The age of these is 
probably the same as that of the neighboring major faults, since they 
present what appear to be the same characteristics in respect to amount 
of dissection, agreement of outcrops on the two sides, and relation to the 
pre-faulting topography. 
One of the two faults lies in the St. George block, a short distance 
east of the town of Washington. Its course is south-southeast for about 
ten miles from the foot of the Pine Valley mountains to Fort Pierce. 
Its northern end was not closely examined, but toward the south it was 
seen to lessen gradually until it passed into a well-rounded anticline, 
which in turn seemed to flatten out entirely where it meets the Fort 
Pierce monocline nearly at right angles. The fault is best exposed close 
to the Virgin river, where its throw is at a maximum. It here crosses 
the diagrammatic breached anticline of Leeds (p. 219), and has dropped 
the west side so far that for a distance of two or three miles the Shina- 
rump ridges are buried in silt brought down by the river. An interest- 
ing feature of this displacement is that its point of maximum throw lies 
