448 CHARLES R. KEYES 



Comanche beds of Early Cretacic Age are both well displayed farther 

 to the eastward near the Texas line. 



General structural features, — The broader tectonic features of 

 the region, as shown in the geologic cross-sections, offer some sug- 

 gestive considerations regarding the forces which have been at work 

 at the southern extremity of the Rocky Mountains. There are four 

 cross-sections within the limits of the Estancia Plains that are par- 

 ticularly instructive. They may be called the northern section, the 

 Cerro Pelon section, the typical plains section, and the southern 

 section. 



The northern, or Santa Fe, geologic cross-section extends from 

 the Thomas Peak of the southern Rockies and a few miles southeast 

 of the city of Santa Fe, in a southwesterly direction, to the Sandia 

 Peak. The distance is 50 miles (section A-B of Group, Fig. 10). 

 This section passes through the laccolithic dome of the Cerrillos Hills* 

 In the main, the synclinal character is preserved between the Rocky 

 Mountains and the Sandia Range. The blue Carbonic limestones 

 are well displayed near both ends of the section. In the middle, a 

 little to the south of the line of section, the Hmestones are also brought 

 to the surface. All of the central part of the trough is occupied by 

 Cretacic formations which are made up chiefly of sandstones. 



The Cerro Pelon section trends nearly east and west (section 

 C-D) . The section is marked by two very pronounced faults which 

 subdivide it into three nearly equal segments. The central portion 

 owes its irregularities largely to the laccolithic disturbances of the 

 neighborhood, the Ortiz group being on one side and the Tuertos 

 group on the other. At the west end the section is monoclinal in 

 character, there being very little if any rising of strata before the first 

 fault is reached. Within this segment a short distance north of the 

 section line several deep-drill wells have been put down. 



The middle segment contains the Cerro Pelon, a sharp shoulder 

 which is a feature of the landscape for a distance of many miles around. 

 This hill rises 600 feet above the plain to the east. On its eastern 

 face is a fault-scarp. The prominence of the elevation is due chiefly 

 to the fact that it is capped by a plate of hornblendic andesite which 

 has a thickness of over 400 feet. This capping-plate dips to the 

 westward and is soon covered by the yellow sandstones of the Cre- 



