The So-C ailed Socorro Tripoli. — Herrlck. 137 
tions. The greatly preponderating portion is of igneous rock, 
such as trachyte, rhyolite, andesite, basalt, etc. The hills on 
the east side of the river in this vicinity are, however, com- 
posed of stratified rocks of Carboniferous age, including some 
600 to 700 feet of sandstone and dolomitic limestone with 
some interbedded gypsum and anhydride. At the foot of this 
expctsure, and nearer the river, is an eruptive dyke not j^efc 
studied. There is everywhere evidence of volcanic displace- 
ment. On the western side of the river is a very interesting 
series of moniliform eruptions along a subordinate axis, giv- 
ing rise to flows of andesite, trachyte, rhyolite and a great 
variety of contactual varieties, now being studied. Among 
these flows in the special region under discussion, there is a 
notable absence of the glassy and vescicular phases of vulcan- 
ism. There are no less than six craters or cores within a few 
miles of this locality, mostly of a distinctly acid nature, the 
most basic flow observed being an augite-andesite, and this 
only at the base of a trachyte core. Yet in only one instance 
have we encountered a glassy lava of the acid series. This 
obsidian is rather remarkable in itself and is of a creamy whi:e 
color with few microscopic inclusions and remarkably free 
from microlites. It tends to be scoriacious and exhibits 
beautiful flowage structure. An analysis made by Mr. Leo 
Suppan of the Socorro School of Mines is as follows: (Num- 
ber 146 Socorro series, south end of the Socorro mountains.) 
Si02 72.052 per cent. 
Al.pd'y 10.812 " 
MgO 1.121 '• 
CaO 9.6 " 
Fe Trace. 
Alkalies Not determined. 
This occurrence and the high percentage of silica of the 
scoria is suggestive in explaining the "tripoli" deposit to 
which we now pass. This area is evidently a minor basin 
within the fluviatile sands. The depression cannot have been 
very extensive or deep and must have been at least tempora- 
rily connected with the river waters or a lagoon. The upper 
stratum in the adjoining terranes is a loose pebbly (u-ag re- 
posing on sand and clay beds of undoubted fluviatile origin. 
Within the basin, which may originally have been half a mile 
in diameter, the ordinary crag is replaced by a very peculiar 
