Block Mountains in N. Mexico. — JoJnison. 137 
This great fault along one prominent plane, however, was 
only one of a large series of parallel faults more or less close- 
ly associated. How many such faults lie buried beneath the 
alluvium to the west is of course miknown, but just west of 
Albuquerque, at a considerably lower level than the Carbon- 
iferous limestone beds at the base of the mountains, are ex- 
posed easterly dipping sands which have been referred to the 
Tertiary by Dr. Herrick ; while still further west, on the same 
level with these sands, the Cretaceous is shown. The contact 
between the Tertiary and Cretaceous is interesting in this con- 
nection, as it is along a broad fault belt, the belt being char- 
acterized by a beautiful set of easterly dipping blocks faulted 
along a series of parallel planes. 
But this faulting was not confined to the region west of 
the scarp. While, as stated above, the eastern slope of the 
mountain is coifiparatively smooth, a closer examination shows 
that it is broken by a series of blocks faulted along planes 
parallel to the main escarpment, as well as along vertical 
planes more or less nearly at right angles to these. In fact, 
the whole eastern slope is covered with duplications, in min- 
iature, of the entire great uplift. As these faults are usually 
normal, the summit of the range is really formed by those 
blocks of this smaller series which have dropped down (com- 
paratively) the most. The essential figures are shown in the 
diagram (fig. i.). 
This same structure is shown still more beautifully in the 
Alagdalena mountains, seventy-five miles further south, where 
the development of extensive lead-silver mines has afforded 
excellent opportunity for the study of detailed features. Here 
again the range consists of a great monoclinal uplift, whose 
abrupt granitic escarpment faces eastward, while the more 
gentle slope of the limestone beds is toward the west. When 
seen from the northwest, the beautiful series of the smaller 
limestone blocks stand out with diagrammatic clearness. The 
faults are in most cases normal (I have seen a few 
exceptions) so that the series of these blocks which have 
dropped down (comparatively) the most, are the highest in 
the range. As in the case of the Sandias, these blocks are also 
faulted along lines more or less nearl\- at right angles to the 
main series. 
