1 86 Bulletin of Lahoratofies of Denison University [Voi.xi. 
and there are few interesting mineral accummulations even from 
a mineralogical point of view. 
In all probability the Sandia fault was only one of a series 
of faults all with a dip to the east. It is probable, for example, 
that the axis of weakness indicated by the series of basaltic vol- 
canoes marks the position of another fault. And the line sep- 
arating the Tertiary from the Cretaceous upon the map is also 
another such line, as will be seen. A result of this system of 
monoclines is to leave a depressed area that may have formed a 
Tertiary sea or estuary occupying the entire width from the foot 
of the Sandia mountains to the west bank of the Rio Puerco. 
In the soft Tertiary strata of this sunken area the two rivers had 
no difficulty in excavating their Pleistocene and their present 
valleys. 
The Rio Puerco Valley. 
The valley of the Rio Puerco, after passing along the west- 
ern foot of the Nacimiento range and skirting the west side of 
the Prieta mesa enters the sheet from the north-west in the 
midst of the Cretaceous area. It would appear that the lava- 
covered Prieta mesa was so far protected from erosion as to pre- 
serve a large series of strata which dn the part of the valley 
covered by this sheet have suffered erosion and removal. 
On the eastern border of Prieta mesa is another valley 
which receives numerous tributaries from the east forming deep 
and narrow gorges with precipitous sides cut in the hard sand- 
stones of the Cretaceous. 
Whether this east Prieta valley occupies a monocline must 
be left in doubt but a mile or more east of the eastern escap- 
ment bordering this valley the nearly horizontal strata are 
broken by a sharp monocline, one of a series of approximately 
north and south faults, found on the west side of the Albu- 
querque mesa. The break is sharp and in many places the re- 
sulting scenic effects are quite picturesque. The same fault 
continues southward along the Rio Puerco and as a result the 
strata about San Francisco, for example, dip to the south-west 
on the west side while on the east side of the river they dip to 
the south-east or east. The discordance seems to fade out to 
