336 
MINING GEOLOGY 
native gold, newspapers made much of the possibility of a gold 
deluge. To those who are most familiar with this golden carpet 
of the desert the increase of world output from this source alone 
promises to assume appalling proportions. 
Although it is not probable that all of the rich gold deposits 
are yet exploited it is quite likely that not many very notable 
new ones remain to be discovered. The great supply of the 
future appears bound to come from the large, but low-grade, de¬ 
posits which, because of hitherto lack of adequate methods of 
handling them, escape attention. The desert tracts of earth prom¬ 
ise best results. There is still another and newer phase of the 
problem. This is the extraction of the gold content from the 
igneous rocks themselves, since they seem, in many cases, to be the 
original sources of the metallic salts. Not all igneous rocks appear 
to yield a definite metallic content. Those masses which are 
known to geologists as laccoliths thus far afford the best returns. 
In this respect laccoliths have a very great advantage over other 
igneous masses because of the fact that they are intruded magmas 
— peculiar bodies which have thick overburdens of strata that, 
as cooling and consolidation take place, prevent the metallic 
vapors from escaping into the air and being lost. 
A notable laccolith, or group of these masses, is the Sierra 
del Oro in southwestern United States, not very far from the 
Mexican boundary. The early Spanish settlers of the region 
gave these eminences the title of Gold Mountains because of the 
fact that around their flanks existed extensive and very rich 
placers. In that day the name was applied to many localities. 
It has long since vanished from our maps. In pioneer days large 
quantities of native gold were obtained from this place. Being 
situated in a great desert the water supply was severely limited; 
and working was onerous in the extreme. Since a method of 
“dry-washing,” or wind treatment, was devised operations on a 
large scale now seem perfectly feasible. 
A new interest now awakens in the Gold Mountains. It turns 
out that the rich gold placers, of which they are the center, are 
only a relatively small part of their metallic wealth. As demon¬ 
strated by numerous assays the mountain mass itself is shot 
through and through with gold. Two of these mountain peaks 
alone show a gold content of more than fifty billions of dollars, 
