20 GROVE KARL GILBERT— DAVIS tMw<MM f$uxxt 



The alternations of coal and limestone in this section is an anomaly. Not less so is the absence of underclay 

 in the southern coals. There should be some discoverable reason. Is it not that the Cretaceous coal is 

 made from plants so far different from those of the coal measures that they have a different soil or even habitat. 



In the same year the best of a very few examples of general discussions was written east 

 of the Kaibab on November 15, concerning the broad denudation of the plateau country: 



The detached mesa (of chocolate shales apparently) that we see beyond [south of] the Colorado . . . bears 

 on the problem of denudation. It is an outlier 20 miles at least from the main bed. Such instances are ex- 

 ceptional & while they indicate great denudation do not solve the problem whether the entire Kaibab region 

 has been covered by the banded sandstone or by the yellow sandstone or by the Cre. & Ter. with the facts now 

 at my command I do not see how to solve it. Where cliffs of like age & character face each other on opposite 

 sides of an anticlinal it is not difficult to bridge the chasm in imagination, but where successive cliffs involving 

 10-1200 feet of strata from Ter down face the metamorphic zone of Arizona & drain toward it, it is hard to 

 say how far they have extended. There are other island mesas of different beds but I can recall none so far 

 removed. 



Some examples are then cited from the Great Basin province southwest of the plateau 

 area, regarding which it is said: 



These however have been separated first by convulsions & only secondarily by denudation. 



Other examples on the plateaus are added, and of these it is said: 



The margins of the several strata are remarkably simple & suggest pelagic erosion rather than fluvial, 

 but there is no other evidence tending the same way. No trace of the denuding coast phenomena appears. 

 Perhaps it is better to suppose that the general limits of denudation are the limits of rapid denudation deter- 

 mined by changes in the character of the beds subjected to it. 



Convulsions, pelagic erosion, and limits of rapid denudation are encountered rarely if at 

 all on subsequent pages. 



It is not a little surprising to find that records of theoretical views as to the origin of the 

 basin ranges are almost wanting. The few that are entered will be quoted in the chapter devoted 

 to that problem, although they are very indefinite as to the main point involved, namely' the 

 occurrence of master faults along or near the base line on one or both sides of the ranges. It 

 might be inferred from this that the discussion of faults was an unfamiliar matter to the re- 

 corder, but the accounts of certain dislocated blocks in the plateau province show clearly 

 enough that such was not the case. One of the most explicit of these accounts concerns the 

 displacement, now well known to many observers, of the Vermilion and other cliffs near Pipe 

 Spring, west of Kanab, on the Utah-Arizona line. There, under date of October 25, 1872, 

 the observed topographic and structural facts are shown in sketches and in plan; and the 

 place of the inferred but invisible fault is made clear in two cross sections. Referring to one 

 of these the statement is made: 



It is evident that Pipe Spr is precisely on the fault & is determined by the abutting of the banded sand 

 against the chocolate (just here slate) shales. 



The cliff on the west of the fault, composed of beds dipping at a moderate angle toward 

 the fault line, decreases in height toward its cut-off termination; 



& pipe spring is at its extremity receiving its discharge from the dip in a manner that astonishes. 



But there is a significant difference between the well-defined faults by which the northern 

 part of the plateau province was found to be divided into huge blocks and the inferred faults 

 by which the basin ranges were thought to be limited. The faults of the first group were proved 

 by standardized and generally accepted evidence, furnished by the repetition of a clearly 

 exhibited series of identical strata on their two sides; whde the faults of the second group were 

 adventurously inferred on unstandardized physiographic evidence, furnished by visible strata 

 only on one side. This evidence was not then clearly formulated even in the mind of its dis- 

 coverer, and was entirely unknown to geologists in general. Indeed it was not explicitly 

 formulated by its discoverer himself until about 30 years later. 



A BOAT TRIP INTO THE COLORADO CANTON 



Something of Gilbert's activity and courage in his first year of western work may be learned 

 from a passage in Wheeler's own report of the venturesome penetration of the Colorado Canyon 

 upstream in the autumn of 1871, thus reversing the course of Powell's boat journey down the 



