Geology of Albiiqjicrqiic, N. M. — Hcrrick. 29 
the several eruptives more definitely in view of the relation 
already noted with human remains preserved beneath them. 
The possibility that these isolated flows may differ greatly 
in age is not to be forgotten, though the essential lithological 
and structural similarity of the rocks as well as the similarity 
of the methods of occurrence suggest a close agreement in 
age. 
Having thus briefly indicated the environs or limits of the 
river deposits we may turn our attention to the latter as they 
are represented at Albuquerque. These beds are exposed at 
various points in the immediate valley of the present river. 
The banks formed by the erosion of the older flood plain are 
often as high as seventy-five feet above the present river bed 
below the city. This plain is a large mesa extending from the 
bluffs to the foot hills on the east and the surface is nearly 
flat but with a gentle upward slope to within a short distance 
of the mountains whence the inclination to the base is more 
rapid. The uppermost of the deposits of this mesa and that 
which lies at or near the surface where un-eroded is a curious 
yellowish-white marl, which is apparently about six feet thick 
and can be traced at about a constant level in the immediate 
river bluffs to a point south of Isleta, as well as north to Albu- 
querque and eastward to within a mile or two of the base of 
the mountains. South of the arroyo formed by the outlet of 
Tijeras canon in the Sandias the mesa is immediately under- 
laid by this deposit so that it is thrown out by burrowing ro- 
dents. A curious feature of this plain is the occurrence of 
numerous slight depressions thickly distributed over the area. 
These are rarely more than five yards across and commonly 
are from eighteen inches to two feet deep and are provided 
with a raised border. They might be taken for buffalo wal- 
lows were it not for their great abundance and general dis- 
tribution. These depressions seem to hold water as they 
sink to the level of the marl. This marl, which may hence- 
forth be known as the Albuquerque marl, has been detected 
on the western side of the Rio Grande as far south as Belen, 
thirty miles south of Albuquerque. The composition of the 
marl is largely calcareous, though it contains small pebbles 
also. Near the city there has been so much erosion that its 
horizon could not be reached for some distance beyond the 
