Block Mountains in Xezv Mexico. — Keycs. 21 
which once formed one face of these mountains. They do 
not seem to be parallel with the great fault. If the main fault 
plane on the w-est face of the Sandias be considered as running 
exactly north and south, then of the small faults on the back- 
slope those at the north end of the range strike southwestward- 
ly ; while those at the south end of the range strike northeast- 
wardly. Only in the middle of the range are these small faults 
exactly parallel to the great fault. Moreover, there is in all 
of the mountain ranges mentioned a system of minor faults 
running transversely to the axis of the range and nearly at 
right angles to the small faults of the backslope. In other 
words, the comparatively insignificant faulting of the back- 
slope is tortional in nature. It appears to have arisen in the 
attempt of the brittle limestone forming the backslope to ad- 
just itself to a complexly warped surface produced by the ma- 
jor faulting. 
Xow the Sandia range is a type of mountain geographically 
known as a Sandia. In Spanish the word means watermelon. 
In general appearance it is as if a barrel floated in the water — 
that is, with one end just submerged and the other end elevat- 
ed. Such, when viewed at a distance of thirty miles, is the ap- 
pearance of the Sandia mountain range rising out of the vast 
plains around it. 
Mr. Johnson says: "This same structure (numerous par- 
allel faulting planes) is shown still more beautifully in the 
Magdalena mountains, seventy-five miles farther 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 towards the west. When 
seen from the northwest, the beautiful series of the smaller 
limestone blocks stand out with diagramatic clearness." 
Now I shall not ascribe to the author just quoted the dis- 
covery of diagramatic clearness with which the blocks men- 
tioned stand out, when viewed from the northwest. I well 
know the pride with which the people of the camp have long 
pointed out, from a distance of several miles, this evidence of 
block faulting on the backslope of the Magdalenas. I have 
gone over the ground with them and have demonstrated to 
