138 TJie American Geologist. 
March, 1903 
In this limestone series there are two zones that have prov- 
en rich in lead-silver ores. It was the block-faulting which 
made the ores accessible, and it is this same faulting which has 
caused endless trouble in following out the ore chutes. Again 
and again the ore is cut off by a fault and the miner must 
work up the fault plane until the ore zone is again located in 
the adjacent block, or else tunnel through barren rock until 
that zone is reached. Besides the faults producing these 
blocks there are innumerable smaller ones with Init slight dis- 
placement. 
It would seem, then, that in the cases cited the total dis- 
placement was brought about, not by a single fault, but by a 
very large series of parallel faults, — a series not to be num- 
bered by tens or hundreds, but more probably by thousands, 
if we take into consideration the vast number whose displace- 
ment is so slight as to be apparent only to one tracing out with 
care some mineral vein. Nor do I believe that we can leave 
wholly out of account the progressively smaller movements 
which have for their idtimate expression molecular displace- 
ment. Along some plane, however, the strain has been greater 
in proportion to the ability of the rocks to resist, and the dis- 
placement along such a plane has been correspondingly great- 
er. To this fact we owe the existence of the mountain form. 
From the standpoint of structure, this mountain form muit 
be considered as purely accidental. It is easy to conceive that 
if we have a series of strata yielding to extreme pressure, the 
displacement might be somewhat equally distributed along 
various parallel fault planes, giving us typical block-faulting 
in which the several blocks (a. b, c, etc.) would be compar- 
atively small, and no mountain form would be produced, 
(fig. 2.) Instances of this nature are frequently seen in the 
field. 
Now if we conceive the displacement along the several 
planes to have been only one-third as great as represented 
above, except in the case of some one fault plane as x, where 
we may consider all the additional displacement to have taken 
place, w'e have the conditions represented in fig. 3, where a 
true monoclinal mountain form is produced. 
This is exactly the condition w^e find in the cases of the 
Sandia and Magdalena mountains above referred to. 
