256 
GREAT BASIN OVERTHRUSTS 
essential feature of Basin Range structure in New Mexico, or 
overthrust evidences was not deterring and did not discourage 
constant outlook for clues to such phenomena. Very early in the 
New Mexican observations it was at one time thought that the 
solution to the problem had been revealed in some of the ranges 
bordering the Rio Grande, and especially in the Sandia Range 
where Gilbert had estimated the displacement of the mountain 
front to be 7,000 feet; but the data soon proved inconclusive. 
In the meanwhile Death Valley came in for examination. The 
recent railroad construction in this region supplied data never 
before accessible. Spring Mountain Range, lying to the east of 
the Death Valley, seemed to offer some good evidences of thrust 
movements, some of which when once made out were discernable 
from Las Vegas twenty-five miles away; but the structures proved 
to be relatively too old to be involved in the present mountain 
building. R. B. Rowe, one of the Government field men, also 
noted the presence of an overthrust in this range, but made no 
record of its significance in Basin Range geology; and Spurr, who 
later used Rowe’s field notes, merely transcribed them without 
comment. 
About this same time the boraciferous deposits near the Virgen 
River came in for examination, and undoubtable evidences of 
thrust movements were found in the face of the Muddy Moun¬ 
tains. It was the observations here that led to the announcement ^ 
that in the orogenies of the Great Basin the major faulting of the 
region might not be of the gravity type as so commonly assumed, 
but of overthrust character. In support of this suggestion, as a 
general proposition, there were a number of considerations pre¬ 
sented besides the ordinary theoretical one. Record showed that 
some of these thrust-planes had been observed but had been mis¬ 
taken for unconformity lines. On the other hand the recognition 
of unconformity plane was doubtless one reason why so few 
fault-lines were recorded bounding the mountain blocks. The 
overpowering influence of the normal fault idea had evidently 
much to do with the general misinterpretation of Basin Range 
orogeny. 
The Muddy Mountains are particularly instructive. The west 
1 Science, N. S., Vol. L,, p. 413, 1919. 
