i8o Bulletin of Laboratories of Denison University [Voi, xi. 
To the southward the series consists of a multitude of iso- 
lated peaks like those of Peralta, Los Lunas, Isleta, La Joya, 
Socorra, San Marcial, and others. Much time has been expended 
in the effort to determine the precise age of these basalt sheets 
and the results seem to be unambiguous. The fact that these 
lavas flow over the bases of the trachyte and rhoylite mountains 
and flows as at Socorro, and burst through and are interbedded 
in the tufa sheets as at the Cochiti district shows the basalt pe- 
riod to follow the trachyte period of eruptive activity. Direct 
superposition on the Tertiary sands in numerous places indicates 
their Post-tertiary age. Often the Tertiary strata are much 
altered and reddened by the contact, being baked and indurated 
in those places where the flow was thickest but less altered by 
the thinner portions of the sheets. The question as to the peri- 
od that may have elapsed since these flows is more difficult of 
solution. We have so far failed to find an instance where the 
lava has flowed over the river deposits of supposed Pleistocene 
age. Wherever the river valley encroaches on the sheets of 
basalt the talus is smoothly removed for the most part. A very 
interesting instance is that of the Peralta volcano which is situ- 
ated on the east side of the river' opposite Los Lunas. Here 
the cone must at one time have been covered under the river 
deposits. No vestige remains of the lava sheet and the neck 
itself has been partly uncovered by later stages of the water. 
On the other hand, in those cases where the cones have burst 
through the Tertiary only they spread out in large sheets. It 
has been repeatedly stated that these lavas are of recent date 
and that they cover remains of human industry. So far as 
this portion of the territory is concerned this may be emphatic- 
ally denied. Specimens of maize imbedded in what was pre- 
sumed to be lava have been displayed in proof of the statement 
that man existed prior to these lavas. It is not denied that re- 
cent igneous flows have occurred in various parts of the West, 
but it seems very improbable that even the latest of these basalts 
could have been cotemporaneous with man in New Mexico. An 
analysis made by Mr. D. W. Johnson of the so-called lava con- 
taining corn proved it to be highly acid and to have a composi. 
