554 Notices respecting New Books. 



flat, straight-planed edges of these strata, Carboniferous quartzites 

 (underlying massive limestones) rest unconformable*. Elsewhere, 

 higher up (near the junction of the Marble Canon, the Little 

 Colorado, and the Grand Canon), the Carboniferous strata, about 

 4000 feet thick, are seen to lie unconformably on Archaean (schistose 

 and granitic) rocks. 



The north side, then, of the valley of the Colorado of the west 

 consists of a series of enormous steps, — rising at first suddenly from 

 the bed of the river in its deep channel, 3000 feet, — then from the 

 bordering esplanade (two miles wide), with a rise of 1500 feet ; and 

 from a great platform of Carboniferous strata about 30 miles wide, 

 by a succession of broad platforms and their scarped edges, to the 

 high Tertiary flats about 50 miles from the Carboniferous plateau. 



The strata whose denuded edges constitute this tremendous stair- 

 case of successive terraces have probably an entire thickness of 

 about 10,000 feet; the surface of the Eocene platform, however, 

 is only about 6000 feet above the Carboniferous plateau, owing to a 

 slight dip to the north throughout, with local increments at the 

 base of each set of cliffs. The slight northward dip is recognized 

 also in the Carboniferous strata south of the Colorado. 



That the enormous mass of material once occupying, as stratified 

 rocks, the wide area bordered by the terraces and escarpments, of 

 which our section above-described is a typical example, has been 

 removed by denudation, Capt. Dutton and his colleagues have ably 

 proved. Also that it was denuded during Tertiary times, and by 

 successive instalments, is well argued out. 



Studying the Canons of the great river from west to east, the 

 Surveyors divide its course geographically into several parts. 1st. 

 Prom the great step at the Grand- Wash fault to the Hurricane 

 fault and ledge is the Sheavwits plateau, with its basaltic fields. 

 2nd. The TJinkaret plateau, bearing Mount Trumbull and other 

 volcanic cones and coulees t, reaches to the Toroweap fault, which 

 is well marked at the Grand Canon. 3rd. The wide monotonous 

 Kanab plateau, with a great lateral canon. 4th. The picturesque 



* The Carboniferous series here consists of: — 



iCherty limestones, \nKc\ft 

 Impure limestones, ) 

 Hard sandstones (including 180 ft. 

 ^2000 it\^ 1 of cross-bedded sandstone). 250ft. 



^ '■ ( Lower Aubrey beds. Thin-bedded red sandstones, nearly 



1000 ft. 

 Walls of the , p e d-wall erour> £ Hard, brown-red sandstones, 325 ft. 



inner gorge ( "™ F r ^ Deep-red massive limestones, 



(with 400 ft. 1 1800 ft. 



of Silurian &c. / Base of the Carboni- Red-brown massive sandstones, 

 =3025 ft.). ^ ferous. 500 ft. 



t The volcanic rocks of these two plateaus are basaltic, and of two 

 different ages, widely separate : one late Miocene or early Pliocene ; the 

 other probably only a few centuries old. 



