g 6 Btilleiin of Laboratories of Denison University [Voi. xi. 
opposite Albuquerque yet there has* evidently been an exten- 
sive erosion of the area separating them for the sandy layers 
below the lava are in places exposed for 450 feet. They con- 
sist of grey and red sands with only occasional indurated bands 
and are covered with sheets of vast extent but usually not more 
than twenty five feet thick. The thicker portions evidently 
were nearer the volcanic centre as the sand and clay below has 
been burnt and indurated and has a bright red color. 
•The prevailing color is reddish, especially in the lower 
parts. An exposure in the mesa now in question to the west 
of Los Corrales and perhaps five miles north of the Albuquer- 
que flow has the same red color but is here capped by the marl 
which is not encountered beneath the sheets north of Bernalillo. 
The entire region north of Bernalillo extending west to beyond 
Santa Ana and extending east and north along the west bank of 
the Rio Grande to beyond the Indian village of San Filipe is 
occupied by this formation and is covered by vast sheets of 
basalt from craters lying to the northwest. One of these has 
been dissected for examination by erosion and was described as 
the Bernalillo volcano in a previous paper. ^ 
Careful search has so far disclosed no organic remains be- 
neath any of these flows though many miles are exposed to view. 
The sheets are at various levels and suggest the probability of 
successive superincumbent layers but, so far, there seems to be 
no evidence of more than one general period of eruption, for 
most of the flows at different levels can either be traced to dif- 
ferent craters or are connected with other parts of the sheet by 
declivities or separated from them by displacements of later age. 
A great deal of base-level undermining is going on and even 
the small streams at once wear down to the general lower level. 
There are a few sand dunes upon the western margin of the 
mesa and it is likely that the major part of the local accumula- 
tions here and there found on ,top of the marl is of the same 
origin. The mesa proper is bare of trees but supports good 
grass as the marl is a water-bearer. To the north as the valley 
^ Amer. Geologist, July, 1898. 
