332 The American Geologist. June, !90o 
lor district. Another part, the portion covered by the United 
State contour map, called the Albuquerque sheet, is described 
somewhat in detail in a report now being printed simultane- 
ously in the bulletin of the University Geological Survey and 
in the Bulletin of Denison University. 
Only a few words then will be required to introduce the 
reader to the field covered. By reference to plate VIII it will 
be seen that Albuquerque is situated in the flood plain of the 
Rio Grande, flanked on the east by a pleistocene mesa rising 
to the foot of the high escarpment of the Sandia mountains. 
These mountains are formed by a monocline, the western foot 
of which is marked by a succession of faults. The Carbonifer- 
ous, which on the eastern side of these faults is found at the 
summit of the range, is found on the west side of the fault at 
ihe general level, and toward the south the red series of Per- 
mian and Jura-triassicage are also at the surface of the mesa. 
To the northward, in the northeastern corner of the map, is 
the great mining district of the Cochiti mountains, in which a 
core of augite andesyte, or diabase, is surrounded by a great 
sea of trachitic tufa with obsidian and brecciated phases. The 
andesyte is the ore-bearing rock, w^hile the tufa is like a great 
barren wall about it. The tufa extends to the Rio Grande 
and in its soft faces are some of the most interesting clifif- 
dwellings in the west. Thousands of these dwellings attest 
ihe fact that the region was at one time densely inhabited. 
The tufa is penetrated at various places by lava flows of recent 
basalt. Especially is this true of the region at the northern 
mouth of White Rock canon, where it would appear that the 
river that at one time made its way through the Santa Fe 
marls, along the edge of the tufa, was blocked by the basalt 
and set back to form a great estuary or lake. It may be that 
at the time immediately following the period of basalt eruption 
the existence of such dams was so general as to have an ap- 
preciable efifect on the climate. At any rate there are several 
instances of such lava dams. In the Rio Grande, below Albu- 
querque, we find evidence of a very extensive dam, to which 
we ascribe the accumulation of stagnant or slow-flowing water, 
by whose agency the pleistocene deposits of the valley, espe- 
cially the secondary marls described in the American Geolo- 
gist, July, 1898, were deposited. The tufa has a very char- 
