28 The Atnerican Geologist. 
July, 1898 
miles. Along this river front the basalt varies from six to 
twenty feet in thickness and is quite vesicular, especially at the 
top, and it reposes on substantially horizontal beds of sand- 
stone of the same age apparently as that forming the mesa 
back from the river, though there are one or more benches 
of river drift nearer the river. So far as seen, the material 
capped by the basalt does not contain the trachyte, andesyte, 
rhyolyte or basaltic pebbles occasionally found in the river 
drift, though it contains quartzyte and limestone fragments 
and also fossil woods. This point is worthy of careful study, 
for by this means it may yet prove possible to ascertain the 
age of the eruptives which are so characteristic of the region 
further north and south. At any rate, the evidence so far at 
hand seems to indicate that the lava flows here are older than 
the river gravels, since we have yet to find a place where the 
lava overtops the fluviatile formation, while near Isleta we 
do find the river gravels overlying the basaltic flows. Much 
depends upon the correct estimation of the age of these lava 
flows, inasmuch they are said to cover ancient pueblos, and in 
some cases maize has been found in a charred state below the 
lava. Near Isleta, an Indian pueblo about twelve miles south 
of Albuquerque, there is another such a flow and the under- 
lying sandy layers have here been greatly indurated and red- 
dened. In this case there are at least fifty feet of the Creta- 
ceous (?) sandstones and shales below the lava, but the eleva- 
tion is not uniform, so that near the southern edge the lava — 
there only about eight feet thick — is covered in places by the 
river gravels. 
Near the northern end of this flow there is an insular 
patch of the lava cut off from the main flow by a narrow valley 
of erosion which must have been at one time the channel of 
the river or an arm of it, and this at a time when the river 
was somewhat higher than at present. 
These facts with others, the details of which need not be 
given here, make it apparent that the local basaltic lava flows 
have occurred at a date more recent than the Cretaceous de- 
posits beneath them and at a period earlier than that of the 
river gravels. Whether the latter are of approximately 
Champlain age or not must be left to the future to determine, 
but it is a matter of no small interest to ascertain the age of 
