334 T^J^^ American Geologist. June, 1900 
passes south. Opposite Albuquerque is a smaller volcanic 
outpour forming five prominent cones overtopping the mesa. 
It seems that the lava does not rest on the marl and that the 
latter may be of later origin, though this cannot be absolutely 
affirmed. Should this prove the case it will throw interesting 
light on the age of the lava and the marl. Adjacent to the In- 
dian villege of Isleta and especially opposite Los Lunas are 
other similar flows of basalt. These are parts of a great system 
extending the length of the territory and everywhere preserv- 
ing the same essential relations. The western margin of the 
Tertiary mesa is scarcely less abrupt, it being due to the ero- 
sion of the Rio Puerco waters and base-leveling extending 
from the flood plain of the river. The western boundary of 
the Tertiary is, however, further west and is formed by a very 
prominent and extensive fault line along which the upper 
Cretaceous in its lignitic division (Upper Fox Hills) is brought 
into abrupt contact with the Tertiary. Nearly the entire west- 
ern part of the area represented upon the map is covered by 
Cretaceous strata. In the northern and south-western por- 
tions of our map, however, the underlying strata appear. First, 
in the northern portion. Here the Jemes or Nacimiento 
range rises with a solid core of granite and othoclase por- 
phyry, shouldering its way through the Carboniferous and 
Permian strata and presenting a bold front to the west, while 
in its northern portions it rises by an almost imperceptible 
angle to the sudden granite escarpment of the western ex- 
posure. Between the tufa and trachyte of the Cochiti range 
and the Jemes ridge flows the Jemes river, a considerable 
creek fed from springs rising at Perea and the Sulphurs still 
further north. Both these localities are well worth careful 
study from a geological point of view. At Perea is an outlier 
of the granite rising through the Permo-carboniferous lime- 
stone and forming a dam in the course of the natural drain- 
age. The underground flow rises m a score of natural springs 
of highly carbonated water, each with individual peculiarities 
adapting it to special uses. The main spring has formed a 
"soda dam" across the narrow valley through which the water 
from above has cut a narrow channel. The dam is chiefly 
travertine, the term soda being derived from the idea that 
"soda water" ought to precipitate "soda." The limestone 
