Art. V.] Herrick, Geology of New Mexico. 1 1 3 
mountain, the former, so far as observed, being composed al- 
most exclusively of granite rocks, while only the northeastern 
part of the San Pedro mountain is granitic. The latter will af- 
ford an interesting field for study as it is essentially an old ande- 
site crater with more or fewer parasitic acid flows. The Car- 
boniferous lies upon its southwestern flank and has been sub- 
jected to so great metamorphism that the lead content has been 
segregated and considerable mines of carbonate exist in the 
neighborhood of the town of San Pedro. The foot hills near 
Golden on the northwest of the range are of Carboniferous 
limestone also but in the valley separating San Pedro mountain 
from the Ortiz group is a tongue of Cretaceous sand and shale. 
The crater proper occupies the entire center of the San Pedro 
mountain, the rim being broken at the northeast at the locality 
of the Saint Lazarus mine and mill. 
West of the Saint Lazarus mine a section of the wall of the 
crater is as follows : 200 feet of sandy and calcareous material 
highly metamorphosed and serpentinized. This may have been 
an earthy limestone with sandy bands. The dip is into the cra- 
ter, i. e. to the northeast. Next is 100 feet of altered lime- 
stone like the lowest horizon, then a sheet of andesite aphanite 
lying conformably above the sedimentary and serving highly to 
metamorphose it at the contact. At this contact is a certain 
amount of copper stain which has excited the curiosity of the 
prospector. The dark band is visible for miles and has univer- 
sally been called the “black lime” by the miners. Above the 
aphanite is a variable amount of acid rock — perhaps a felsite. 
The south and western rim of the crater are quite sharp and 
are essentially similar tnough more of the andesite is found in 
places. In the interior the andesite is more in evidence and is 
mixed with acid flows giving rise to the conditions for gold col- 
lection. As a matter of fact there is gold in the placer gravel 
within the crater practically everywhere. The contacts of the 
andesite and the acid rocks are also habitually auriferous and 
are worked to a considerable extent, the “Gold Standard” 
mine being quite well known locally. The northern wall of the 
crater is broken by a massive dyke extending north and south 
