54 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
old age in a cycle of erosion that was introduced by a relatively remote 
and wide-spread deformation. The low ranges with sprawling, fad- 
ing spurs and wide open valley mouths are regarded as the resid- 
ual reliefs of the earlier cycle, undisturbed or only slightly disturbed 
by the later faulting, and not greatly modified by continued erosion. 
The high ranges, with strong slopes, simple base lines on at least one 
side, and relatively narrow valley mouths, are regarded as uplifted 
and tilted blocks of the previously eroded region, now well entered 
upon a new cycle of erosion. The intermont depressions, more or 
less aggraded, appear to be relatively depressed areas, now covered 
with waste from the higher areas; but the possible occurrence of bev- 
elled rock floors at a small depth beneath the waste slopes of the low 
ranges must not be overlooked, especially where the waste slopes 
extend far up towards the low mountain crests. 
Gilbert has explained the Basin ranges essentially as tilted blocks; 
his original account gave much attention to the faulting of the moun- 
tain blocks, little attention to their pre-faulting structures, and still less 
attention to the pre-faulting and post-faulting erosion that they have 
suffered. Spurr has more recently explained the Basin ranges without 
regard to block faulting, as the result of “compound erosion,”” that is, 
of a long continued series of deformations and associated erosions, 
but without attempting to specify the sequence, the dates or the rela- 
tive values of the processes concerned. ‘Thus stated the two explana- 
tions of the Basin ranges seem incompatible. 
Gilbert’s later discussion of the Basin range problem, in the oral 
presentation of his paper at Washington, as above stated, takes fuller 
account of the three chapters that his theory suggests; namely, the 
pre-faulting structure and erosion, the faulting of the mountain blocks, 
and the post-faulting erosion. His conclusions are well supported by 
Louderback in a recent Bulletin of the Geological Society of America, 
concerning an isolated range in the Great basin, in which it is clearly 
shown that the pre-faulting deformation was followed by so long a’ 
period of erosion as to reduce the district to a surface of small relief, 
which was then covered by a lava sheet; that a block of the compound 
mass was then uplifted with a tilt to the eastward; and that later ero- 
sion has not yet accomplished extensive dissection of the tilted block. 
In this case, the term, monoclinal block, is justified by the attitude of 
the tilted lava sheet, but not by the disordered structures beneath it. 
The incompatibility between Gilbert’s and Spurr’s explanations 
disappears if the various factors of Spurr’s compound erosion are 
given specific values. Modern faulting is not excluded from these 
