94 
Bulletin of Laboratories of Denison University 
[Vol. XT’ 
U 
THE AREA WEST OF THE RIO GRANDE. 
The triangular area between the Rio Puerco and the Rio 
Grande is occupied by a nearly level mesa between 700 and 
900 feet higher than the river at Albuquerque. This field is 
nearly level but inclines to the southeast with the inclination of 
the strata composing it. The dip is apparently not more than 
ten feet to the mile. The mesa is about three and three-quar- 
ters miles wide at a point opposite Albuquerque and increases 
toward the north as the two rivers diverge. Nearly west of 
Albuquerque is the group of recent volcanoes previouly de- 
scribed.^ The cones lie on the eastern margin and consist of a 
group of three major and as many smaller outlets. The three 
larger craters rise to about three hundred feet above the mesa 
or one thousand feet above the river. The material is a dark 
basalt which was emitted in small amounts intermittantly. The 
upper surface is vesicular and exhibits the usual appearance of 
a thin sheet which has cooled rapidly. It extends to but a 
short distance on the west over the level mesa but extended 
much further eastward, spreading over the somewhat lower 
levels in a sheet not generally more than 20 feet thick. It 
formed a lobed and irregular area which has been little eroded 
since its outflow. The materials underlying are, so far as can 
be seen, the same as those forming the mesa and not those of 
the river valley, though it appears that there was some erosion 
before the eruption, perhaps by a flow of hot water as in other 
cases in this neighborhood. This group of cones is only one 
of a series of basaltic post-tertiary volcanoes which can be traced 
along the entire length of the Rio Grande in New Mexico. 
Professor R. T. Hill has described the later volcano flows 
of New Mexico in general terms in the Bulletin of the Geol, 
Soc. of America, III, p. 98. In concluding this paper he says: 
“ It is also evident that eruptive activity has occurred in the 
Texas-New Mexican region from Cretaceous to the present 
1 Geology of the Environs of Albuquerque, American Geologist, July, 1898. 
