io6 The American Geologist. February, 1903. 
oes are "plugs" or "necks" ; but in a few cases both the outer 
wall and the inner plug still remain ; and in the case of one of 
the larger craters, side craters, forming lateral vents, were 
seen. 
These craters are situated on the mesa to the west 
of the Rio Grande, near San Felipe, for which the writ- 
er named the group. The southernmost crater observed 
is about two miles to the southwest of the village. The craters 
of this group are in a line, curving to the west along the great 
dike. They rise in altitude, one above another as they appear 
farther to the northwest till they reach the summit of the mesa 
where the largest one of the group is to be seen. Its plug rises 
6500 feet above the sea, and its diameter is more than a half 
mile in one direction and a mile and a quarter in another. To 
use the words of our Indian guide ; "Esta padre de todas," 
it is the father of all. From it toward the west the craters 
become less and less in hight till they are reduced to the level 
of tbe mesa. The lava flow from these craters covers 300 
square miles in solid mass, and many more miles in detached 
patches. The lava flow is augmented at the west by the flows 
from the Cochiti volcanoes and by the flows from Mt. Negro 
(Sierra Negro). It reaches the Pueblo of Jemez on its west- 
ern limit and covers almost the entire space in the triangle be- 
tween the Rio Grande and the Jemez river to the south of 
Canyon Honda. The lava sheet is from twenty to one hundred 
and twenty feet in thickness. It is of the basaltic type ; and, 
on account of its broken down, rough; condition, it is called 
"Malpais." 
Date of Bruption. — The age of the lava flows is only con- 
jectured. Dr. Herrick first supposed that the underlying strata 
which are red and grey sandstones, were of Cretaceous age, 
and so stated in his "Geology of the Environs of Albuquerque," 
New Mexico, (Bull, of the University of New Mexico, V. I, 
pp. 40-43). In this paper on the environs of Albuquerque he 
says : 
"These blufifs (the bluffs that border the Rio Grande 
on the west) afford the most instructive illustration of 
base-leveling that could be imagined. The basalt reposes 
on the red and grey Cretaceous strata, which here remain near- 
ly horizontal. The sources of these flows is not difficult to 
