Art. IX.] Herrick-Johnson, Gcology of the Albuquerque Sheet 19 1 
Puerco valley at the point where that river enters the sheet. 
This remarkable mesa we have called in our notes the Island 
Mesa (Mesa Isleta). Passing northeastward from the Mesa 
Isleta one enters a picturesque confusion of eroded and disman- 
tled bluffs of Cretaceous shale and sand. Some four miles dis- 
tant is an excellent exposure of the contact with the Tertiary. 
The fault is very marked. The Cretaceous strata are tilted to an 
angle of about 45 degrees and the upper Cretaceous sandstone 
is brought to view with its abundant fauna, Above this is a 
yellowish sandstone and below it bands of lignite and gypsifer- 
ous shales. The monocline is very well marked and a part at 
least of the Tertiary strata seem to be involved in the up-lift. 
The red beds of the Tertiary bluffs on the west side of the 
Albuquerque mesa may be traced to the monocline without 
change of level but here they are abruptly titled. Under them 
and lying in contact with the inclined beds of the Cretaceous 
are beds of soft white sandstone with remains of vertebrates 
which we presume to be representatives of lower Tertiary strata 
than those elsewhere exposed to view. It is evident that the 
later stages at least of the movement causing the anticline took 
place after the Tertiary and this exposure therefore gives us no 
clue to the original relation between the Tertiary and the 
Cretaceous. 
This monocline in some adjacent places exhibits less of the 
evidence of disturbance, the slip being abrupt with little altera- 
tion of level in the strata on either side. The Tertiary red sands 
are curiously modified in the neighborhood of this monocline 
for there are enormous areas covered with concretions of small 
and uniform size often in botryoidal masses. A few miles south- 
west of the south-west corner of the Zia reservation the lignites 
are exposed by the erosion of the Tertiary. The details of dis- 
tribution of the surface exposures have not been studied nor 
have the economic conditions but the lignites are no doubt the 
same as those at the Isleta Mesa monocline farther south and 
there has been a considerable disturbance and dislocation which 
may be referred to the influence of the axis of the Nacimiento 
range. 
