Art. IV.] Herrick, Geology of New Mexico. 91 
merge into the trachyte. The obsidian is in some cases por- 
phyritic with crystals which have so far decomposed as to baffle 
identification but are probably sanadin. Similar beds are seen 
at various points further west. Above the obsidian is an ex- 
tensive flow of trachyte in which in some places flowage phe- 
nomena bear record of its motility while in others it seems to 
have crystalized quietly in situ. Quartz trachyte passing into 
rhyolite is apparently found near the top of the series here as 
elsewhere. The presence of the explosive type of lava might 
be taken as evidence that the mountain was itself the theatre of 
eruption yet such flows often extend many miles and we have 
found a curious obsidian breccia in a trachyitic or rhyolitic 
magma at Texas Spring at the distance of at least six miles 
from a possible source. 
Magdalena mountain stands, therefore, as a natural dissec- 
tion giving a vivid conception of the earlier geological condi- 
tions of the region. Passing now to the region immediately 
north of the main range, we encounter a series of trachytic erup- 
tives of which “Nipple Mountain” is the first and others con- 
tinue the series to the extensive trachyte crater of Bear Moun- 
tain. To the east of this range the Carboniferous limestone lies 
in the floor of the plain instead of being tilted as in the Mag- 
dalena range. Small quantities of lead are found in the veiny 
and faulted limestone and dykes of “ felsite ” apparently of the 
trachyte age passing westward into the great andesite region 
north of the town of Magdalena seem to have filched the lead 
to fill in these east and west veins. This andesite area or re- 
gion of basic eruptives is interesting as probably forming a part 
of the product of the first Magdalena eruption and probably 
contains more than seventy-five square miles and may be much 
more extensive. It includes the Cat Mountain Camp and ex- 
tends northward perhaps ten miles, while to the south, as already 
seen, it is for the most part covered by the later acid eruptives. 
The area is circumscribed to the east and north by the trachytes 
of the Bear Mountain series and on the west by the southward 
extension of the Gallinas to the Tres Montosos. At many 
places trachyte and rhyolite patches remain in situ above the 
