162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the prefaulting surface have been depressed out of sight in the thrown 
block. Advanced old age in the prefaulting cycle, and youth or early 
maturity in the present cycle, are the conditions demanding the least 
measure of block faulting; for the small relief of advanced old age in 
the preceding cycle would be consistent with the easy burial of all rock 
surfaces near the fault line in the thrown block ; and youth or early 
maturity in the present cycle would call for the least addition to the exist- 
ing height in reconstructing the crest of the mountain block near the 
fault line. 
It is worth while to call attention at this point to a corollary that fol- 
lows from the provisional conclusion above stated regarding the prevail- 
ing absence of modern faults except along the base lines of certain ranges 
where independent lines of evidence lead to the belief that the modern 
movements are but the latest displacements on faults of much greater 
age. The corollary*is this: the total displacement on these long-lived 
faults must be usually greater than the ordinary measure of prefaulting 
relief in the Great Basin region. For the fault lines must have origi- 
nally run indifferently to the structure of the region, and therefore in- 
differently also to whatever reef the region had assumed when the 
faulting began ; and yet the thrown block is now asa rule completely 
covered with gravels and sands washed from the heaved block. Excep- 
tions to this rule are found at certain points, but they are rare. If 
further exploration confirm the provisional conclusion above referred to, 
this corollary may have some value. 
In this connection it may be noted that the great measure of displace- 
ment inferred by King for the Wahsatch fault (b, 745) seems unnecessary. 
If the folded strata of the range had been reduced to moderate relief by 
prefaulting erosion — and this seems not improbable if one may judge 
by the enormous volume of the Eocene (Vermilion creek) Tertiary to the 
east (King, b, 745) — the measure of the fault need not be more than 
enough to raise the crest of the range above the rock floor that is buried 
under the sediments of the Salt Lake basin ; that is, from 6,000 to 
10,000 feet instead of 40,000. 
It was suggested by Van Hise in the discussion of this subject at the 
recent Washington meeting of the Geological Society of America that 
the displacement in faults of large throw, such as those by which the 
Basin ranges have been formed, are believed to be, is usually distributed 
on grouped fractures instead of taking place on a single plane of dis- 
placement. All the ranges that came under my observation last summer 
are non-committal on this point, except in so far as the absence of dis- 
