90 The American Geologist. February, 1903. 
a continuation of them. They can be traced from one to the 
other without break. The Placita formation being Pleistocene, 
these marls are Pleistocene also. Under what conditions the 
Rio Grande marls could have been deposited can only be con- 
jectured by the writer, because his observations did not ex- 
tend far south. It seems probable that at the time the Rio 
Grande series of volcanoes were active, there must have been 
not only a change in the position of the strata at different 
places along the Rio Grande embayment and in the bed of the 
channel itself, but the groups of volcanoes of this series must 
have poured a part of th'eir lavas into the river channel and 
dammed the stream at various places. The first of these hap- 
pened at San Felipe. A spur of the San Dias, forming a ridge 
in a curved line to that village, dammed the channel and caused 
a lake to be formed above the Indian village. The lake con- 
tinued to exist until the river cut the channel where it now 
runs. The present channel at San Felipe must be younger 
than the lava flow inasmuch as it is incised in the latter ; 
moreover had the original course of the Rio Grande occupied 
this area and it had been subsequently flooded with lava, we 
ought to find some evidence of the filling of the pre-historic 
valley but no data favoring this interpretation were obtained. 
In the vicinity of Ysleta however there is some evidence to 
prove that the prehistoric valley of the Rio Grande was com- 
pletely dammed by the lava flows. More data bearing on this 
point, however, are necessary before such a conclusion should 
be fully accepted. 
Resume of the Stratigraphy of the Region. 
At the close of Carboniferous time, or earlier the Jemez 
mountains were uplifted, and associated with their develop- 
ment are to be found large intrusions of granite and porphyry 
occupying an axial position. During the period of mountain 
building, the western flank of the Jemez was faulted off. These 
mountains were subsequently surrounded by a shallow Juras- 
sic sea in which were deposited red sandstones and shales with 
gypsum, to a thickness of 2600 feet or more. Then came the 
Jurassic revolution. The mountains were re-elevated and the 
Jurassic strata to the west of the mountains were faulted and 
tilted to a nearly vertical position. At this time the volcan- 
oes near Pelado and in the Valle Grande countrv became act- 
