on.i.KKT.] ALBUQUERQUE TO FLAGSTAFF. 469 



direction by superior Carboniferous and Mesozoic strata, all dipping 



from the mountain. But going westward from the Archean belt the 

 unaltered rocks are not found in the same order. The tough Carbonif- 

 erous limestone that holds its own so valiantly on the summit does not 

 appear at the west, as it should if the structure of the mountain were 

 anticlinal; but the first strata seen, after passing the valley gravels, 

 which bury the base of the Archean wall, are of Cretaceous age, and 

 they dip toward, rather than from, the ridge. The mountain is a great 

 but simple monoclinal mass, bounded on the west by a profound fault, 

 along the line of which is the river valley. The difference of level 

 between the Carboniferous strata on the crest of the mountain and the 

 dissevered fraction of the same strata, buried far below the Cretaceous 

 rocks in the valley, is not less than 11,000 feet (3,300 in.), and something 

 greater than this must have been the throw of the fault that separated 

 them. 



Thence, westward to Flagstaff and northward to the Grand canyon 

 of the Colorado, the route lies exclusively within the Plateau region. 

 The rocks are Cretaceous, dura-Trias, Carboniferous, and volcanic. The 

 Cretaceous system includes alternations of yellow sandstones with gray 

 argillaceous shales, and there are occasional beds of coal. The maxi- 

 mum thickness is about -4,0(K) feet (1,200m). The Jura-Trias is composed 

 of sandstones with sandy shales and marls, and is everywhere character- 

 ized by brilliant colors. In the upper part of the system lenses of gyp- 

 sum occur, and further west beds of salt are associated with them. At 

 the west, calcareous beds have been found in the upper part, with marine 

 shells, called Jurassic. Farther east, beds near the summit of the 

 series have yielded plants referred to the Trias, and bones referred to 

 the dura. About the Zuni uplift 91 the most conspicuous member of 

 the system is the Wingate sandstone, a massive bed nearly 500 feet 

 (160 in.) thick occurring near the middle of the system, which has here 

 a total depth of about 3,500 feet (1,000 m.). Farther west another 

 massive bed appears near the upper limit, ami acquires topographic 

 prominence. The Carboniferous system is characterized by two great 

 beds of limestone, which weather but slowly and are thus rendered 

 prominent in the topography. The lower appears in the Grand canyon 

 of the Colorado; the upper, known as the Aubrey limestone, is seen in 

 the Zuni mountains, and constitutes a large portion of the plateau 

 traversed between Flagstaff and the Grand canyon. Above this are a 

 few hundred feet of biight-colorcd shales and sandstones, resembling 

 the Jura-Trias rocks, but classed with the Paleozoic by reason of fossils 

 of Permian type discovered in southern Utah. The same Permian 

 fades characterizes fossils of the upper layer of the Aubrey limestone. 

 Associated with these are volcanic rocks with many modes of occur- 

 rence. Basic lavas, chiefly andesitic, rest upon the plateau in great 



