Art. IX.] Herrick-Johnson, Geology of the Albitquerqite Sheet. 177 
either a secondary product of metamorphism or a portion of the 
uneroded cap of the granite forming the axis of the range. To 
the north east the Cochiti range of augite-andesite (diabase) and 
trachitic tufa rises above the Tertiary plain here largely covered 
by a broad apron of recent basalt. Further west the granite 
ridge forming the Nacimiento range projects over the Jura-trias- 
sic and Cretaceous plains but the only influence of this range 
upon our sheet is visible in the fault and monocline which ex- 
tends southward from its southern extremity and forms the east- 
ern boundary of the Cretaceous in the Puerco valley. 
The Albuquerque Mesa. 
The central and largest portion of the sheet is formed by a 
large, low, triangular mesa bordered on the east by the valley of 
the Rio Grande, on the west by the Rio Puerco valley and on 
the north by the valley of the Jemes river. This mesa which 
faces Albuquerque on the west has been considered by Captain 
Dutton a part of the Zuhi plateau but the Captain seemed to 
have but the vaguest ideas of the geography of the country 
he so eloquently described in his monograph of the geology of 
the Mount Taylor region and supposed' that the Rio Puerco 
flowed into the Rio Grande some miles above Albuquerque in- 
stead of at La Joya fifty miles south of it.^ This mesa we may 
call the Albuquerque mesa and the basaltic peaks which form 
its most prominent feature the Albuquerque volcanoes. Oppo- 
site Albuquerque the mesa rises to 5700 feet or 750 feet above 
the river level. The dip of the underlying Tertiary strata is in 
the vicinity of 50 feet to the mile to the south-east so that the 
western rim of the mesa is three hundred feet higher than the 
eastern and at the northern portion the western bluffs attain a 
hight of about 6750 feet above the sea. The dip is greater in the 
northern portion of the mesa, doubtless as a result of the prox- 
imity of the axis of uplift south of the Nacimiento range. 
^ The passage reads as follows : “ There are unfortunately within the lim- 
its of the region covered by the map given herewith two rivers named Puerco. 
One runs north-east of Mt. Taylor and empties into the Rio Grande a few miles 
north of Albuquerque.” It may be added that the geological delineations are 
also incorrect in many particulars. 
