Ayfc. m.] Herrick Johnson, Geology of the Albtiquerque Sheet, 22^ 
forms the bulk of the section. Both series seem to be of anor- 
thite. The augite, as in the preceding instances, is chiefly in 
scattered grains ; although larger crystals are not wanting. 
Magnetic iron is scattered throughout the ground mass; and, in 
a few instances, hematite is encountered. The granular suffusion 
from the ground mass renders a minute study of the rock 
difficult. 
Isolated Trachyte Cones in the Rio Grande Valley. 
The isolated volcanoes of the Rio Grande valley are nearly 
all basaltic and it has usually been assumed that all such minor 
peaks are of this character. After noticing that there are at 
least two classes as regards the relation of these lavas to the 
Tertiary strata of the valley the enquiry was natural whether 
there might not be a distinction in composition betv^een those 
that are found in connection with superficial flows and those 
that have been buried under Tertiary sands and marls. The 
larger ranges like the Socorro and Limitars toward the close of 
their active existence passed through a trachyte-rhyolite stage 
and rocks of the silicious series are found perforating the earlier 
andesites but the apparent relations of the small isolated volca- 
noes spoken of is with the recent basalts and it is not always 
easy to determine a difference between the materials and those 
of altered portions of the basalt corles. 
An instance of such !a buried cone is afforded by a volca- 
nic cone on the west side of the Tertiary mesa west of the Rio 
Grande opposite Albuquerque. This hill has evidently been 
buried by the material of which this mesa was formed and has 
been re-excavated by the Rio Puerco. Unlike the recent vol- 
canoes on the east side of the mesa on the banks of the Rio 
Grande, some six miles distant, this cone is not associated with 
superficial flows but all such evidences of overflow have been 
removed by erosion, apparently prior to the deposition of the 
Tertiary sands and marls. The subsequent denudation has left 
exposed the irrregular neck and boss-like protrusions from the 
base. 
A considerable variety in the rock is apparent to the eye 
