470 GEOLOGICAL EXCURSION TO THE ROOKY MOUNTAINS. 



cones built in Tertiary time an<l left by the progressive degradation of 

 the region upon pedestals of less resistant rock. Next in importance 



are lava Hows. partly andesitic, but largely basaltic, and of such antiq- 

 uity that the country has been eroded about them, leaving them as 

 caps of small plateaus or mesas. In some cases, where the outpoured 



lava has been removed by erosion, the congealed lava of the conduit 

 remains as a volcanic neck. Of more modern date, and often of extreme 

 recency, although not historical, are basaltic cinder cones and basaltic 

 couldes, which diversify the plateaus and course through the valleys. 



Normally the strata are approximately level and interest centers in 

 the variations from this attitude. The Zuni mountains were; produced 

 by a moderate aparching, the axis of which trends northwest and south- 

 east. On the northeast side the dips are gentle J on the southwest 

 steep. From the northwestern end of the uplift, a inonoclinal fold — 

 the Nutria fold 1 " — continues for several miles, and then gradually fades 

 out. Where a monoclinal fold is normally developed it is the equiva- 

 lent of a fault, except that strata are Hexed instead of fractured; in 

 this case flexure and fracture are combined (Fig. 2!)). Farther westward 

 another monocline, the Defiance fold, is seen, as well as minor flexures. 

 and near the Grand canyon yet others. 



The San Mateo Plateau is occupied by an immense composite sheet 

 of lava TOO square miles (1,800 Bq. km.) in area. By the degradation 

 of the surrounding Cretaceous rocks it has received a relative altitude 

 of more than 1,000 feet. Upon it stand numerous cinder cones and 

 the great andesitic mass of Mount Taylor, and around about it a multi- 

 tude of volcanic necks testify to its greater original extent. Mount 

 San Francisco, likewise a great cone of audesite, was built upon dura- 

 Trias rocks, but these have been worn from the surrounding plain, 

 together with all other strata down to the Aubrey limestone, and the 

 talus of audesite almost completely conceals the sedimentary pedestal, 

 so that the peak seems to spring 5,000 feet (1,500 m.) into the air from 

 a plain of Aubrey limestone. All about it are more recent basaltic 

 cinder cones and lavas. 



Where the train enters the Plateau region, soon after crossing the 

 Bio Grande, it follows a valley cut through the Cretaceous and into the 

 Jura-Trias. Near Laguna it rises to the Cretaceous, but before that 

 point is reached a fresh black lava stream appears in the valley, and this 

 is kept in sight for many miles. Thence to McCartys the visible sediment 

 aryrock is all Cretaceous. Northward at a short distance appears the 

 San Mateo plateau, with Mount Taylor on its back, and in the face of the 

 plateau are to be seen folds of Cretaceous strata formed previous to rlie 

 volcanic eruption. Nearer by are several buttes, constituted by volcanic 

 necks, and just beyond Cubero station is a dome-like lava cone. At 

 McCartys another fresh lava stream is encountered in the valley, and this 

 remains in sight until Bluewater is passed. A few miles beyond McCartys 

 the train passes from Cretaceous to dura-Trias rocks, but the character- 



