36 GROVE KARL GILBERT— DAVIS 



The material for study is then summarized under four heads; mountain building by dis- 

 placement, mountain building by eruption, stratigraphy, and erosion. As to displacements in 

 the plateau province — 



faults and folds abound through its whole extent, but they are comparatively of great simplicity. They are 

 indeed so simple that they can be completely known. Their entire phenomena may be comprehended, measured, 

 described and delineated. The course of many a fault can be traced from end to end, and its throw measured 

 at every step. The form of many a fold can be determined throughout, and pictured or modelled in miniature, 

 with every detail of flexure. 



Special attention is given to the monoclinal fold, then a novelty, although in the plateau 

 province it is "a characteristic type of displacement and is rivalled in frequency only by the 

 fault." Similarly, the huge blocks of earth crust, dislocated but undeformed between the 

 flexures or faults, are presented as a class of displacements "so little known heretofore that it has 

 not found place in the manuals of geology." It is believed that when all blocks are studied out, 

 the generalizations reached "will not be inferior in value to any single contribution that has been 

 made to our knowledge of the results of orogenic movements." 



As to mountains resulting from eruption, two types, the Uinkaret and the Henry Moun- 

 tains, are presented in addition to the ordinary volcano. The Uinkaret type, as described by 

 Powell north of the Colorado Canyon near the western border of the plateau country, consists 

 of ancient lava flows, eroded in the form of high mesas and then mantled by later flows "so as 

 to give the appearance at first glance of a range made up entirely of volcanic matter." The 

 Henry Mountains, which Gilbert was then studying and concerning which a full account will 

 be given here in a later chapter, were briefly stated to consist of " a number of bubble-shaped 

 domes, one for each individual mountain of the group." Both these novel types are declared 

 to "diverge most widely in character from those with which geologists are already familiar." 



Stratigraphy is briefly treated. By reason of the numerous deep canyons strata may be 

 examined "not merely along a simple line, but throughout an extended area. With such 

 exposures . . . the history of a system of sediments can be made out with a completeness 

 that surely can not be excelled elsewhere." Yet in spite of their visibility, neither in this article 

 nor in other reports are novel results announced regarding stratified formations. The problem 

 of deposition was little considered as compared to the problem of erosion. This problem, 

 evidently a favorite with Gilbert, is given a detailed analytical treatment quite unlike that of 

 the three preceding topics but closely similar to that of the chapter on "Land sculpture" in 

 the Henry Mountains report, to be summarized below. Perhaps the most significant statement 

 here is that concerning the contrast between the widespread degradation which the uplands 

 of the province have suffered, estimated at 5,000 feet, and the depth of erosion in the narrow 

 canyons, which is of similar amount; but hardly a hint is given of a great movement of 

 elevation between the times of broad degradation and deep erosion. 



