Jcuicz — Albuquerque, N. Me.w — Regan. 69 
General Description. 
This region is in northwestern New ^Mexico between longi- 
tude 106" 20' and 107'' west, and latitude 35" and 36^ north. 
Roughly speaking, it is a triangle with its apex toward the 
south. It is bounded on the southeast by the San Dia moun- 
tains, on the southwest by the Rio Puerco, and on the north 
by the upper plateau of the Jemez mountains. Its principal 
river is the Rio Grande, and its commercial center is Albu- 
querque. The Santa Fe railroad enters this section at the 
northeast near Thornton, and passes through it just to the east 
of the Rio Grande to Albuquerque. At this point the road 
branches, one branch of the system going to El Paso, Texas, 
the other, the Atlantic and Pacific, to California and the Pa- 
cific coast. 
History. 
Some parts of this section have been visited by geologists 
in the last twenty-seven years. Professor E. D. Cope visited 
the region lying between Algodones and Placita in 1875; ^"'^ 
while there h,e found Pleistocene deposits overlying Tertiary 
and Cretaceous rock. In the following year he also visited the 
Puerco marls at the source of the Rio Puerco. Dr. C. L. Her- 
rick has described at considerable length, the southern portion 
of the section in volume I of the Bulletins of the Universitv of 
New Mexico. Among his subjects treated in that bulletin, 
which will receive notice here, are the "Pleistocene marls" 
(here called the Zia marls) and "The Rio Grande loess." He 
also describes some of the volcanoes of this region in the same 
bulletin.^* A further discussion of the above citations will be 
given when the respective areas concerned are considered. 
Structure. 
Nature of rocks. — The rocks of this area are intrusive, 
eruptive, and sedimentary. The intrusive rocks are the cores 
of the mountains of Jemez and San Dia ; and the dikes 
throughout the entire region. They are granites, porphyries, 
gneisses, et cetera. 
The eruptive rocks arc the volcanic plugs, lavas, and tuffs. 
The lavas are basalt, trachyte, and rhyolyte. Obsidian also 
occurs in large quantities on the Jemez plateau. All the erup- 
tive rocks, except the trachyte-rhyolyte lavas, will receive at- 
tention under the subject of "Geography and Physiographv." 
• The numerals thus used (1) refer to references at the close of this paper. 
