92 The American Geologist. February, 1903. 
Below Albuquerque, except during high water, the water seeps 
into the sand and the river bed becomes dry. The river at this 
point runs through a wide inner valley or flood plain on the 
down throw side of the San Dia fault. Here the river indicates 
at least three states of activity. In the early Post-Tertiary it 
cut out a deep, wide channel in the Pliocene rocks. Then it 
refilled thie trough, thus formed, with the Rio Grande marls 
discussed above. Now it is again deepening its channel. 
Age of the Rio Grande. — The Rio Grande at Albuquerque 
is younger than the Pliocene in which it cut its deepest chan- 
nel. But where its channel at San Felipe was during the early 
Pleistocene is unknown. It was not where it is now, because 
the Post-Tertiary lava flows covered the area through which 
the present channel is cut. Had there been a channel at this 
point we should expect to find portions of the lava in its orig- 
inal position, within its boundaries but no evidence of this sort 
was obtained. It is quite possible that the river in early Post- 
Tertiary flowed farther to the east where the Pleistocene de- 
posits are now to be found near Placita and that anticlinal 
uplift which sends a spur north from the San Dias to the east 
of Placita changed the course of the river to its present loca- 
tion. 
The Rio Pnerco. — The Rio Puerco closes in, on the west, 
the lands described in this paper. It is a tributary of the Rio 
Grande, entering that river below Albuquerque. It runs at 
such an angle to the Rio Grande that the lands between them 
form a V-shaped area. Its source is in the Puerco beds at 
Nacimiento west of the Jemez mountains. The Puerco's gen- 
eral course is south. It has no tributaries except near its 
source. In its lower course, it has cut for itself a canyon some 
800 feet in depth. This stream is dry nearly all the year round. 
The Jeniea River.- — The Jemez river like the Rio Puerco, is 
a tributary of the Rio Grande. Its direction of flow is south- 
east. Near San Isidro it branches, and Salt river passes around 
to the west of the Nacimiento mountains. Farther north it 
again divides, one branch goes east of the Red Beds in the 
Vallecieto valley, another west of these beds through Guadel- 
upe Caiion, and the third or middle branch up San Diego 
Cafion, which is chiselled in the Red Beds along a fault line. 
These trubutaries divide and subdivide till their branches pene- 
