Art. IX] Herrick-Johnson, Geology of the Albuquerque Sheet i8i 
tion impossible for basalt or an ordinary slag. It was not till 
our attention was called to the nature of the clay resulting from 
the disintegration of the trachytic tufas in the Bland and Jemes 
district that the matter became plain. We heard much of an 
ancient smelter at the mouth of Pino canon in the Cochiti 
range. After much search we found a portion of an arch com- 
posed of brick which were in some places so completely vitrified 
that the broken surface could be told from obsidian breccia in 
the adjacent range only upon careful inspection. Further 
search proved that the so called prehistoric smelter is a portion 
of a brick kiln burned a few years ago to supply a stamp mill in 
the neighborhood and that the project had to be abandoned be- 
cause of the vitrification of the brick. Still later we were able 
to duplicate the supposed lava with the corn imbedded near the 
village of San Isidro in the Jemez valley where either the Pueblo 
potter had the same misfortune or a fire in the granery not only 
charred the grain but fused the adobe and imbedded the grains 
therein. At any rate the artificial origin of the “ lava ” is per- 
fectly plain. 
The thickness of the lava flow from the Albuquerque vol- 
cano is rarely more than 25 feet and the inclination of the sheet 
is sharply toward the river and in one place toward the north 
end of the sheet it appears that there was a flow over the decliv- 
ity formed by the bank of the then existing river, as a fragment 
of the flow occupies a lower level and is nearly buried in the 
sand of the flood plain. To the west the flow extended but a 
short distance from the craters. The northern border of the 
mesa is formed by the valley of the Jemez creek which is 
wholly excavated out of the Tertiary sands. The declivity is 
gradual and exhibits extensive and irregular erosion, while on 
the west side the mesa often presents a rather abrupt escarp- 
ment with fantastic battlements and bastions. 
The mesa is entirely without water except such as is 
retained by the marl from recent rains but this suffices to sus- 
tain a good growth of gramma grass and accordingly affords 
grazing for herds of horses from the ranches in the river valleys. 
In those portions of the mesa where the marl has been eroded 
