Art. IX.] Herrick-Johnson, Geology of the Albuquevque Sheet 179 
of the Scorro mountains are made up of trachyte and rhyolite 
fragments and the coarseness of the material diminishes with the 
distance from these \sources. No estimate is now possible of the 
entire thickness of the Tertiary as seen in the exposures of this 
sheet. West of the mesa in the Rio Puerco valley we may se- 
cure sections like the following; White marl, 25 ft.; gray sands 
and gravel, 25 ft.; red sand, 40 ft.; whitish sand (unknown); 
hiatus; concretionary yellow sandstone, 25 ; flags and sand, 50 ; 
yellow flags, 5 ; gray sand and gravel, 65 ft.; coarse gravel. 12 
ft.; pinkish crag, 40; crag with basalt (fault?) 25 ft.; crag, 50 
ft.; lime sinter, 25 ; pinkish sand with flags, about 200 ft.; red- 
dish brown concretionary sandstone, 35 ; white concretionary 
sand, 25 ; yellowish-pink sand, 75 ft.; white sandstone, un- 
known ; fault, beneath which are sands and lignites of the 
upper Fox Hills group. 
The portion of the mesa lying within the limits of our 
sheet is broken by only one intrusive, that which forms the 
Albuquerque volcanoes above mentioned. These basaltic cones 
or necks are prominent objects from the valley at Albuquerque 
from which they are distant less than eight miles. The five 
necks plainly evident in the series lie in a north and south line 
evidently along a fracture or line of weakness. It is not diffi- 
cult to determine that this series is but a detached portion of a 
system extending along the entire Rio Grande valley. To the* 
northward is the small Bernalillo volcano described in the 
American Geologist, Vol. XXII, page 40, Still beyond is the 
great San Phillipe lava sheet derived from the craters near the 
south end of the Cochiti range. Still further north vast flows 
occupy the Tertiary mesa south-west of Santa Fe and portions 
of these flows having poured into the narrow valley of the Rio 
Grand at White Rock canon accummulated in terraces and su- 
perposed flows. At the north end of the canon these Post-ter- 
tiary flows may be found penetrating and altering the tufa of the 
east side of the Cochiti range. North of Santa Clara begins 
an enormous flow of similar age and character covering thou- 
sands of square miles. Evidence of a post-tertiary dam and lake 
may be seen about San Ildefonso. 
