ac.dkmy of scen-ces] WHEELER SURVEY 29 



The present altitude of the plateaus is due to a later uplift, after "the main folding of the 

 range system," on fault lines at the western base and a few miles to the east of the Wasatch 

 Range; " along these two lines have been the main movements which have reversed the position 

 of the two systems; placing the plateau above the plains of the Basin Range system" (252, 253). 

 The great erosion of the basin ranges here implied as following their "main folding" will be 

 referred to again in discussing the explanation of the ranges as fault-block mountains. Gilbert 

 was evidently much impressed by the contrast to the Appalachian revolution of the Atlantic 

 slope that was presented by the almost unbroken continuity of deposition in the plateau prov- 

 ince from the "Silurian" to the Tertiary; the chief break in this long sequence is a slight uncon- 

 formity between the clays of the lower Trias, later called Permian, and an overlying conglom- 

 erate: the clays "were somewhat eroded by the current which spread" the conglomerate, "as 

 is shown by the inequality of the surface on which it rests" (175). 



THE GREAT UNCONFORMITY 



Of wider significance is the discussion of the tremendous unconformity at the base of this 

 great Paleo-Meso-Cenozoic series in the plateau province; it had already been recognized by 

 Newberry and Powell, but was more fully discussed by Gilbert as follows: "In the Grand 

 Canon of the Colorado . . . the Tonto sandstone [the formation in which the Cruziana, 

 above noted, occurs] rests directly on the plicated and eroded schists and associated granites, 

 and demonstrates them pre-Silurian." Hence "the Archaean strata had been deposited, 

 plicated, raised above water, and eroded, before the epoch of the Tonto group." The depo- 

 sition of the "Silurian" sandstones with their marine fossils on the underlying crystalline rocks 

 was therefore interpreted in Gilbert's first report to mean that the Paleozoic ocean "slowly 

 encroached upon the Archaean continent, paring its ridges, filling its hollows, and spreading 

 over all . . . the coarse siliceous detritus that constituted the advancing beach" (186, 187). 

 That the encroachment of the ocean was largely due to the subsidence of a preexistent land 

 had been previously pointed out in the statement that the Paleozoic "series, in a great number 

 of instances, exhibits limestone at the top and vitreous sandstone (quartzite) at base, with 

 usually shale between"; and this, following Newberry, is declared to be "the typical sequence 

 of deposits upon a continent slowly sunk beneath the ocean" (183). The amount of sinking 

 which continued from "early Silurian to late Cretaceous," is estimated at "no less than 

 8,000 feet" (187). 



It is interesting to add that, although no mention is made of them in the text, one of 

 Gilbert's sections of the Colorado Canyon wall (p. 184) correctly represents the very ancient 

 but moderately inclined pre-Paleozoic strata, later called Unkar by Walcott, which, repeated 

 by faults in several parts of the canyon, form east-dipping, wedgelike masses, with their under 

 surface resting unconformably upon a remarkably smooth, slanting floor of the still more ancient 

 crystalline rocks, while their upper surface is everywhere obliquely beveled by the great erosional 

 plain, cutting evenly across the moderately inclined Unkar strata and the strongly plicated 

 crystalline rocks alike, which serves as the even floor upon which the Tonto sandstones were 

 so broadly outspread. Gilbert's section here referred to is believed to be the first published 

 illustration in which one of the wedgelike masses is properly represented; the corresponding 

 figure in Powell's Exploration of the Colorado River of the West (p. 212, fig. 79) is seriously 

 incorrect and misleading. 



The great "Pre-Silurian" unconformity is more broadly treated in Gilbert's second report, 

 where its occurrence in the southeastern part of the basin range region is strikingly described: 



The break between the Archaean schists and the Paleozoic beds is strongly marked. The Archaean sedi- 

 ments were plicated, were tilted, and were lifted above the ocean and eroded before the Paleozoic were laid 

 down (510). 



A general statement is made later: 



There are two general facts in regard to the geological history of the great West that deserve especial 

 mention . . . The first is that the pre-Silurian stratigraphical break is as complete and as universal in the 

 West as it is in the Eastern States and Canada . . . And, second, there is always, at the contact, a contrast 



