DAVIS: MOUNTAIN RANGES OF THE GREAT BASIN. 161 
seated faults. Taken alone they might perhaps be so considered ; but 
taken in connection with all the associated features they cannot be re- 
garded as independent of displacement in the underlying rock mass. 
There is, however, one aspect of the modern faulting that deserves 
consideration. In all cases that I have seen, the modern movements 
are so placed that they must be taken as the continuation of long- 
maintained displacements whose total measure must, as a rule, amount 
to many hundreds or some thousands of feet. No other explanation 
has been found for the presence of such mountain masses as have been 
described above, standing in strong relief on one side of the fault line, 
while there are only gravels and sands to be seen on the other side. It 
is, of course, conceivable that modern faulting may have been here and 
there begun on new lines, essentially independent of the older fault 
lines, but such cases must be rare; for it is to be expected that, if a 
modern fault occurs on a new line, it should run across country indiffer- 
ent to pre-existent structures. Such a fault might run obliquely across 
an intermont plain, then traverse a mountain range, and continue into 
another plain beyond the range, the whole length of the fault being 
marked by a scarp of more or less distinct form. The Great Basin has 
not yet been carefully enough explored to prove that no such faults 
occur; but the region is well enough known to warrant the provisional 
statement that new faults of a date as recent as the scarps of the Bonne- 
ville deposits are rare, except in connection with cld faults. 
On the other hand, there seem to be many ancient faults in the Basin 
ranges on which movement ceased long ago. This is shown by the ob- 
literation through erosion of relief due to faulting ; or sometimes by so 
great an excess of erosion in the uplifted block over that in the thrown 
block, that the thrown block now stands above the lifted block, the fault 
scarp being thus topographically reversed by erosion. Many examples of 
these kinds are given by Spurr. It does not, however, seem admissible to 
argue from the absence of modern movement on these faults, or from the 
apparent absence of modern faults within the ranges, that no long-main- 
tained faulting can have taken place along the range borders. That 
must be determined by evidence furnished by the borders of the ranges 
themselves. 
The Measure and Distribution of Faulting. — It may be noted that only 
an incomplete measure of the total movement in block faulting is deter- 
mined by the difference of altitude between the mountain base and the 
reconstructed crest in a lifted or tilted block: for in addition to this 
measure, there must be a certain supplement by which the inequalities of 
