28 GROVE KARL GILBERT— DAVIS CMB * OIM [$St 



STRATIGRAPHY 



A large amount of painstaking routine observation on the basin ranges as well as on the 

 plateaus is represented by 21 columnar sections, with measures of the thickness of successive 

 formations and brief accounts of their composition and fossils, Meek being the authority for 

 most of the species named (157-170). The sections include an immense accumulation of 

 sedimentary formations, the time relations of which, conveniently exhibited in graphic form 

 (171), extend from Archean to Quaternary. These well-generalized records are followed by a 

 brief review of each larger tune division from younger to older (172-186), which constitutes the 

 most considerable contribution to historical geology that is to be found in any of Gilbert's 

 reports. A brief passage of altogether exceptional nature is here included — 



The genus Cruziana was first described by A. D'Orbigny from the Lower Silurian of South America. It 

 has since been found in Lower Silurian strata in France and Sweden; in Primordial strata in England, New- 

 foundland, and Montana; in the Chazy group in Canada, and in the Clinton group (Upper Silurian) in New 

 York.' Its known vertical range is thus entirely within the Silurian and its broadest distribution in the Lower 

 Silurian. By these facts [other fossils also being here referred to] I am let to conclude that the Tonto group 

 [lying unconformably on crystalline rocks at the bottom of the Colorado canyon] is certainly Lower Silurian 

 in age and probably Primordial (185, 186). 



Whether this passage was borrowed from a report of the paleontologist, Meek, or whether 

 it is a reminiscence of the Gilbert's years in Cosmos Hall is not clear, although the first-person 

 pronoun suggests the latter alternative; but in either case the passage is peculiar, almost unique, 

 in making repeated references to foreign localities. Such references are rarely found in Gilbert's 

 reports. In common with other explorers of the West, he was so overwhelmed by the great 

 mass of new facts there discovered and by the heavy labor of description, and discussion that 

 he seems to have found no time for comparing them or their explanations with more or less simi- 

 lar facts and explanations previously recorded by the geologists of other continents; his work 

 was essentially American. 



HISTORICAL GEOLOGY 



Among other results of importance determined by Gilbert as a member of Wheeler's survey, 

 as well as by the geologists of rival surveys, were several generalizations regarding the historical 

 geology of the West as contrasted with the previously established standards of the eastern 

 United States; for example, the meagerness of the Upper Silurian and Devonian beds, the 

 marine origin of the Carboniferous, the occurrence of Cretaceous coal — " Whenever there shall 

 be a market for it, coal will be developed in all the indicated areas of Cretaceous outcrop" 

 (546) — and the supposed lacustrine origin of much of the Mesozoic and Tertiary strata in the 

 plateau province. On the last-mentioned topic an important change of opinion was made 

 20 years later. Curiously enough, the large-scale occurrence and presumably aeolian origin 

 of certain cross-bedded sandstones, best exhibited in what he called the Gray Cliffs, now usually 

 known as the White Cliffs, in the northern part of the plateaus, received little attention. The 

 sandstones were long afterwards referred to as exhibiting " superlative cross-bedding " but with- 

 out a suggestion of origin. 2 Certain paleogeographic changes, indicated by variations of the 

 stratigrapliic column from place to place and presumably caused by ancient deformations and 

 emergences, are discussed, for the most part briefly. Thus among other contrasts between 

 the region of the basin ranges and of the plateaus is the emergence of the former in mid-Mesozoic 

 time, so that since then, although possibly suffering progressive deformation, it has been exposed 

 to erosion, while the latter continued to subside and to receive sedimentary deposits, largely 

 derived from the former, as late as the Tertiary era (63, 1S7). 



The present greater altitude of the plateau province resulted from a later movement when 

 its long-continued subsidence was reversed to upheaval. "The Wasatch and the country 

 immediately east of it [the plateaus] have been elevated, relatively to the adjacent portion of 

 the Great Basin, not less than 4,000 feet since the drainage of the Great Tertiary lake" (59, 60). 

 Gilbert's associate, Howell, made an excellent contribution to this subject: he wrote on a later 

 page of Wheeler's Volume III than Gilbert's first report : 



When the Cretaceous and Tertiary seas covered the present Plateau region, the Great Basin, as it is now 

 called, was the continent which furnished the material for the heavy beds of rock which were then deposited. 



■ Ripple-marks and cross-bedding. Bull. Oeol. Soe. Amer., x, 1899, 135-140. 



