188 



entire area agaiu, it presents two basin-sliaped tlistricts, the Rio Puerco 

 and the Rio Grande, with the strata in each respective basin dipping- in 

 general towai'd its center. The separating line at the north between 

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

 uplift. It is continued at the south in a line of hills which decrease in 

 altitude as they recede from the main range. The two basins 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 south of the Jemez range. There is also evi- 

 dence that the Nacimiento iNIountains Avere, originally, the result of a drop 

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

 subsequently covered in pai't 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. Mes;i Blanco was left an escarpment by a drop on its northern 

 side of more tlian 1,C>00 feet, l.OOO feet of which still remain. The escarp- 

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

 escarpment to the \\est of the same mesa is 1,200 feet. On its western 

 margin the sti-;ita of this mesa dip toward the east at a great angle, and 

 at a greater angle toward the west on its eastern side. Tlie wliole coun- 

 try, as is indicated altove, is exii-emely l)i-oken n\): tlie livers in their 

 process of ))ase-leveling iiave chiseled their channels deeji into the rock. 

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

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

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

 angle ranging from l.'i'' to !M(." In many pl.ices the region is ;i bad land 

 country. A\'iien" llie lava is suixTinijiosed on it. it Is of liic "mal pais" 

 type: and whei'e the lav;i is wanting, especially along the Itreak-lines, 

 ^'mauvaises tt>i'res." The culminating points of the area under considera- 

 tion are, the crest of the San Dia Mountains, the monolith .Mt. ("altizon (»n 

 the Rio Puerco and INIt. Pelado. the culminating point of tlie .Temez INIoun- 

 tains. 



XATFRK OF ROCKS. 



Tlie rocks of tliis region are intrusive, eiaiptive and sedimentary. 



The intrusive rocks are the cores of the res[)ective mountain districts 

 ■of Jenu'z and San Dia, and the dikes throughout the entire area. They 

 are gi-aiiites. ]ioritliyries. gneisses, etc. The eruptive rocks are volc;inic 



