130 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The latest general discussion of the region is by Spurr, in whose essay 
a review of earlier writings may be found. This author concludes that 
“the mountain fronts studied are, in general, not marked by great faults, 
and, conversely, that the ascertainable faults are very rarely attended by 
simple fault scarps.” He therefore rejects Gilbert’s hypothesis and ex- 
plains the Basin ranges as the “ results of compound erosion active since 
Jurassic times, operating on rocks upheaved by compound earth move- 
ments which have been probably also continuous during the same period.” 
It is further suggested that the ranges were probably differentiated dur- 
ing Cretaceous time, when a greater precipitation is assumed to account 
for their dissection: ‘subsequently the climate became arid and the 
water supply was not sufficient to remove the detritus from the valleys, 
which filled up ” (1901, 265, 266). I have elsewhere briefly stated the 
reasons why this explanation seems unsatisfactory (1901, a). 
The Basin ranges have for some years been of special interest in svs- 
tematic physiography, for if Gilbert’s explanation of them is correct, they 
offer unusually simple examples of mountain uplift and sculpture ; examples 
that may be adduced as relatively elementary illustrations of the difficult 
group of mountains in general, and that may therefore be with propriety 
presented to beginners for introductory practice before the description 
of complicated mountain ranges is undertaken. This opinion was con- 
firmed on the appearance in 1884 of Russell’s vivid account of the faulted 
lava-block ranges of southern Oregon ; for these seemed to be even simpler 
and younger than the ranges farther south. Ranges of this kind are of 
a further interest in that they support in a certain measure the more 
primitive theory of mountain-making ; namely, that mountains are the 
immediate results of uplift, comparatively little modified by erosion, while 
the intermediate troughs are the effects of depression : in a word, that dis- 
locations of the earth’s crust are here chiefly responsible for the observed 
relief of the region, and that the part played by erosion is subordinate. 
It is now generally agreed that this primitive theory finds little support 
in such ranges as the Alps, where the existing forms of peak and pass, 
spur and valley, are the product of extensive erosion in a deformed and 
broadly uplifted mass. A recurrence to the older theory in explanation 
of the ranges of the Great Basin is therefore a wholesome discipline. 
It has for some time seemed to me that there was good evidence fer 
regarding the Oregon lava-block ranges as types of the youngest, most 
elementary mountain forms known to geographers, and for placing the 
ranges of the Great Basin in Utah and Nevada as types of larger and 
more maturely sculptured ranges, appropriately following the introduc- 
