DAVIS: MOUNTAIN RANGES OF THE GREAT BASIN. 163 
coverable fractures in the front part of the mountains requires that any 
additional fractures besides the one which determines the mountain base 
should be forward from it, and concealed under the gravels of the pied- 
mont plain. Certainly if there are distributed faults in the Spanish 
Wahsatch block, the displacements on the minor faults within the block 
must have ceased long enough ago to have been obliterated, so far as 
surface form is concerned, in the smoothly graded slopes of the spurs ; 
while the movement on the main fault along the front margin of the 
block has continued to so modern a date as still to have distinct control 
over the form of the terminal facets. 
An article of interest in this connection has lately been published by 
Mr. D. W. Johnson on block mountains in New Mexico, from which it ap- 
pears that Sandia mountain, near Albuquerque, is a large block with its 
chief displacement along the strong escarpment that it presents to the 
west, but with many smaller displacements within its mass, thus con- 
firming the suggestion of distributive faulting as made by Van Hise. 
The dynamics of faulting are, however, not yet so well understood that it 
is safe to assert the occurrence of distributive faulting in all block moun- 
tains. Surely no one could have hesitated to believe that the Sandia 
block was faulted, even if minor faults had not been found on its back 
slope. In the plateau province of Utah and Arizona several of the greater 
faults are demonstrably on relatively simple fractures; for the strata of 
the adjoining blocks come close to the fault line without noticeable dis- 
turbance ; there is room for fault breccias fifty or one hundred feet wide, 
but apart from that the faults seem to be for the most part simple and 
clean cut. 
It is evident that the examples of Basin ranges here described are 
alone too few in number to support any safe conclusion as to the origin 
of the Basin ranges in general. The Wahsatch range forms the eastern 
border of the Great Basin province, and although it seems to be of fault- 
block origin, it cannot be taken as a typical example of one of the isolated 
ranges within the Great Basin. The chief profit that comes from the 
study of the Wahsatch range is the definition of certain criteria by which 
fault-bordered ranges may be determined elsewhere ; and this profit will 
be increased when the back or eastern slope of the Wahsatch shall have 
been studied in the same relation. The other ranges above mentioned 
have, however, a certain value in that they were chance samples, not 
selected beforehand because they were believed to be faulted blocks, but 
observed as they happened to be passed while the observer was on his 
way to other points. They thus serve to indicate at least a probability 
that other ranges in the same region have a similar structure. 
