136 The American Geologist. M'^^''^^, 1903. 
plane would explain the phenomena observed, allho the ex- 
istence of other parallel faults was regarded possible. In some 
of the cases cited by Gilbert the existence of several such 
fault planes was shown. 
The presentation of these papers, and the discussion which 
followed, recalled to the writer's mind some few observations 
made in the mountains of central New Mexico during- recent 
years, and while they may add nothing- really new to what is 
already known of block-faulting, they are here recorded as of 
possible passing interest. 
Twelve miles east of the city of Albuquerque rises the 
abrupt western escarpment of the north and south range of 
the Sandia mountains, to a hight of over five thousand feet 
above the level of the city, or over ten thousand feet above 
sea level. The range consists essentially of a great monoclin- 
al uplift of granite and metamorphosed sedimentary rocks, 
capped by easterly dipping limestones and sands of Upper 
Carboniferous age. The western face is steep and rugged, 
presenting a very bold and striking topography. The eastern 
slope is very gentle, and comparatively smooth and unbroken. 
The western base of the range is fairly regular along the 
north and south line, altho this regularity is somewhat ob- 
scured by the advanced stage of erosion and the great alluvial 
fans which extend well up into the canons. 
At the base of the western escarpment occurs, exposed in 
several places, limestone from which Dr. C. L. Herrick col- 
lected a few poorly preserved fossils of Carboniferous age. 
This limestone on the western side of the great fault plane 
appears to be the equivalent of the beds capping the range 
on the eastern side of this plane, and indicates a fault of more 
than three thousand feet. A few miles further south, along 
the base of this same monoclinal uplift, we find the Red Beds 
of still later age, exposed just at the base of the escarpment. 
It is thus evident that the great movement which gave to the 
mountain its precipitous western face was along this one 
plane, or at least along a series of parallel planes confined to 
so narrow a zone as to constitute, for all practical purposes, 
but one single plane. No evidence of more than one such 
plane was seen. 
