188 



entire urea a.iiam. it presents two I'asln-sliaped districts, tlie Rio Fuereo 

 aud tlie Rio Grande, witli the strata in each respective basin dipping in 

 general toward its center. The separating line at the north l)etMeen 

 these basins is the Xaciniiento Mountains, the west wing of the Jemez 

 uplift. It is continued at tlie soutli in a line of hills which decrease in 

 altitude as tlicy recede from the main range. The two liasins merge 

 into one below Albuquer(iue. The whole area is faulted and much broken 

 and high escarpments often still mark the fault lines. Examples of such 

 escarpments are the San Dia Mountains, Mesa Blanco, and one on each 

 side of the Red Beds just soutli of the Jemez range. There is also evi- 

 dence that the Nacimiento Mountains were, originally, the result of a drop 

 on their western side. The resulting escarpment has been worn down and 

 subsequently covered in part by sedimentations that it is not so strong 

 in relief as the San Dia escarpment: the Carboniferous strata which flank 

 this range on the east are entirely wanting to the west of these moun- 

 tains. Mesa Blanco was left an escarpment by a drop on its northern 

 side of more than 1,000 feet, 1,000 feet of which still remain. The escarp- 

 ment to the east of the Red Bed mesa is now 900 feet in height and the 

 escarpment to the -nest of the same mesa is 1,200 feet. On its western 

 margin the strata of this mesa dip toward the east at a great angle, and 

 at a greater angle toAvard the west on its eastern side. The whole coun- 

 try, as is indicated altove, is extremely broken up; the rivers in their 

 process of base-leveling have chiseled their channels deep into the rock. 

 Great dikes and numerous volcanoes puncture the strata; and lava-flows 

 -cover hiuidreds of square miles of its surface. The dip of the whole re- 

 gion. Avhen a dip is noticeable, is usually away from the mountains at an 

 angle ranging from l."t° to !)0.^ In many places the region is a bad laud 

 ■country. "Where the lava is superimposed on it. it is of the "mal pais" 

 type: and where the lava is wanting, especially along the I)reak-lines. 

 "mauvaises terres." The culminating points of the area under considera- 

 tion are, the crest of the San Dia Mountains, the monolith Mt. Cabizon on 

 the Rio Puerco and Mt. Pelado, the culminating point of the Jemez Moun- 

 tains. 



XATT'RK OF ROCKS. 



Tlie rocks of tins region are intrusive, eruptive aud sedimentary. 



Tlie intrusive I'ocks are the cores of the respective mountain districts 

 •of Jemez and San Dia. and the dikes throughout the entire area. Tliey 

 are granites, jiorphyries. gneisses, etc. The eruptive rocks are volcanic 



