256 The America')! Geologist. AprU,i897 
THE GEOLOGY OF A TYPICAL MINING CAMP IN 
NEW MEXICO. 
Hy C. L. Herrick, Socorro, N. Mox. 
Plates XIII and XIV. 
The Magdalena mountain range forms a comparatively iso- 
lated district and illustrates many of the conditions of min- 
eral accumulation, as well as a variety of geological problems 
exceptionally well. Situated about twenty miles west of the 
Rio Grande valley, it forms a secondary axis parallel to the 
major one which must at one time have occupied the site of 
the present valley. The Magdalenas are situated upon the 
margin of the belt of Carboniferous limestone and sandstone 
strata so' well seen at various points in the valley. As in the 
case of the other north and south ranges, the present moun- 
tain axis is near the base of the original anticline and exhib- 
its the uptilted strata, here dipping with an inclination of 
about 45'^ to the southwest. The principal intrusives are 
found near the synclinal axis and contribute to the ore con- 
centration. 
The range, so far as studied, is about thirty miles long and, 
like all other axes of uplift in this region, is interrupted at 
intervals by local craters. In the present case the northern 
portion is relatively less disturbed, while the southern three- 
fourths is chiefly volcanic. The mineral association in the 
two portions obeys different laws and affords instructive evi- 
dence of the constancy of these principles under similar con- 
ditions. It may be premised that in this portion of New 
Mexico the sequence of eruptives has been from basic to acid. 
The older flows being usually andesitic, the following trachy- 
tic, while the latest are rhyolites. True, there are basaltic 
flows of a still later age, producing mat jna's and recent cra- 
ters, but this does not alter the general fact that the flows 
chiefly concerned in outlining the present geotechnic of the 
region obeyed this rule. Again, it is necessary to note that 
the stratified series is chiefly sandstone below and limestone 
above, so that in each case of uplift the lime has suffered dis- 
placement, while the subjacent sandstone has frequently been 
greatly metamorphosed even to the extent of transformation 
into gneiss or granite. In a general way it may be said that 
the northern portion of the range, being less fundamentally 
