Geology of Albuquerque, N. M. — Herrick. 27 
To the east of the city, at a distance of about ten miles, the 
eastern wall of the valley is formed by a rather abrupt escarp- 
ment of gneisses, granites and quartzytes in what may have 
been the axis of a monocline. Near the base the abrupt as- 
cent is broken by rounded foot hills, and the products of ero- 
sion from this granitic metamorphic series are to be found 
widely distributed in the valley gravels. Respecting the age 
of this formation it is only known that it is older than the 
coal measures, limestones of the latter age being everywhere 
present at the top of the metamorphic rocks.* In some places 
there are pebbles of granite in the silicious layers at the base 
of the Carboniferous, and unconformity may be assumed in 
many places, while the resemblance of the silicious strata in- 
ierbedded in the limestones to the upper parts of the meta- 
morphic seiies and the lack of apparent unconformity in some 
places suggests that the interval may not have been so long 
as has been supposed, or may occur lower in the series — a 
suggestion also made possible by the occurrence in the same 
relative position in the granite series at Limitar (fifty miles 
south) of a band of fossiliferous limestones at a depth of sev- 
eral hundred feet below the lime beds of undoubted Carbon- 
iferous age. The metamorphic series east of Albuquerque 
rises in places to a hight of 2,000 feet above the mesa or 300 
feet above the river at the city. The dip is a few degrees (15- 
25) to the east, so that the irregularities of erosion as well as 
the varying amount of uplift brings the Carboniferous beds at 
some places much nearer the mean level. If the mesa east 
of the river is really in the axis of an anticline, as has been 
suggested, it must be admitted that the western wall has been 
entirely removed at this place. If, on the other hand, the 
fold was a monocline the whole western portion has been 
faulted hundreds of feet below the river level. Nearly oppo- 
site Albuquerque the river gravels are much obscured by sand 
drifted from the mesa to the westward which is underlaid by 
sands of a relatively late period (Cretaceous). 
From five small volcanic cones a small sheet of basaltic 
lava has spread over an area of about twenty-five square miles 
with an irregular .front upon the river of about five or six 
*We have been unable to verify the reports of fossils older than the 
Carboniferous in any part of the Rio Grande valley. 
