22 
The American Geologist. January, i904. 
their full satisfaction that at least some of the principal fault 
planes bounding some of the so-called blocks are not fault 
planes at all. The phenomenon is produced by differential 
weathering of thick alternating beds of hard limestone and soft 
shale. The backslope slants about thirty degrees. Moreover, 
after going through the mines lower down on the mountains, 
where many minor faults are to be noted in the tunnels, the 
slips were found to be all too insignificant to even show their 
presence on the weathered mountain slope. 
As I take it, the physiographic history of the region is es- 
sentially as follows : The alteration in central New Mexico of 
narrow mountain ridges and broad plains presents some fea- 
• tures which are not easily understood until the regions both 
to the eastward and westward are taken into consideration. 
In both directions the basin character of what is known as the 
bolson plain is soon lost. The plains become confluent and the. 
mountain ranges more disconnected and finally altogether is- 
olated. Still beyond, the plain alone exists without notable 
mountains. This condition persists on the one hand to the gulf 
of California, and on the other to the gulf of Mexico. 
At the beginning of Tertiary time the region between the 
two great gulfs north to what is now the Colorado line must 
have been a vast lowland plain, with but faint relief features. 
A large part of this plain was on the bevelled edges of Creta- 
ceous and older strata, as is well shown now in its remnants 
still clearly discernible. The Las Vegas plateau, the Llano Es- 
tacado, the bolson plains of central New Mexico and some of 
the less broken plains of eastern Arizona seem to belong gen- 
etically together. To the east and west a broad submarine 
platform was formed from the sediments derived from the 
planing ofif of the central land area. When the general bowing 
up of the region took place later in the Tertiary, the great plain 
formed was partly a peneplain of destructional land origin and 
partly a constructional plain of marine origin. 
In the uprising, however, faulting took place on a large 
scale, giving rise to the numerous monoclinal block mountains 
in the region now within the boundaries of New Mexico. 
There were various halts in the general uprising tendencies and 
the Mesozoic and the youngest Paleozioic beds have been 
stripped ofif the mountain summits. Two or three times the 
