34 The American Geologist. July, isos 
of a yellowish color and containing few large pebbles, but a 
larger number of what seem to be fragments of granitic de- 
bris. The upper portions especially contain numerous white 
chalky grains that may be simply the kaolinized remains of 
the feldspathic fragments. Although the oldest of these beds, 
the loess nevertheless contains fragments of unaltered wood 
in some places. The upper surface of this Rio Grande loess 
is irregularly eroded and upon this eroded surface there re- 
poses a layer of shingle pebbles of considerable though in- 
definite thickness. It may be roughly estimated at an aver- 
age of twenty-five feet. The upper part of this bed of Rio 
Grande gravel is variable in nature, but everywhere in the 
vicinity of Albuquerque where the uppermost deposit is pre- 
served it is found to support a band of white chalky material 
which we shall call the "Albuquerque marl." 
The present gorge of the Rio Grande in the vicinity of 
Albuquerque is not more than two or three miles wide in most 
places and this small flood plain is dotted with ranches and 
villages. From the west at several places the flood plain is 
encroached on by abrupt lava-capped blufifs. One such is 
adjacent to Isleta and extends from a point just west of the 
pueblo for three to four miles northward. The first thought 
is that the fiow forming the cap has taken place since the river 
gravels were deposited, for the material covered is a sandy 
and horizontally stratified deposit not unlike the finer forms of 
river detritus. The source of the lava, which is a black, 
rather vescicular basalt, is near at hand, being a small volcanic 
cone about three miles northwest of the pueblo and not more 
than three to five hundred feet above the water level of the 
river. The eastern wall of the crater has been broken away 
and the latest flow has accordingly been toward the sea, in 
which direction it may at one time have extended in long 
radiating streams from the main sheet as far as the present 
river bed, though these outlying portions have long since 
been carved down by the river and only great piles of debris 
remain, and the greater portion of this material is doubtless 
buried beneath the sands. The blufif remaining looking to- 
ward the river, is exceedingly irregular, but in hight is quite 
constant, being about lOO feet at the portions lying to the 
north but sinking to less than thirty at the southern limit. 
