34 GROVE KARL GILBERT^DAVIS [U ™ mR \vo?xxt 



upcurving the superficial strata" (504). In spite of the small vertical distance of the sub-Zuiii 

 rock transfer, the magnitude of the transfer far exceeded that in the companion volcanic eruption; 

 the reconstructed Zuni dome was calculated to have a volume of 700 cubic miles and the sub- 

 Zuni rock transfer must have been of similar measure; but the volume of the Mount Taylor 

 cone was estimated to be only about 60 cubic miles. 



An example of Gilbert's graceful phrasing follows: Although "assumed by earlier explorers 

 to be a continuation of the Sierra Madre of Mexico . . . the Zuiii range, far from deserving to 

 be entitled the mother of a family of mountains, is a lonely orphan, dissevered from all kindred. 

 It stands, in the midst of the plateau region, a mountain of upheaval; from every side of it 

 the strata stretch in level tables. ... It is truly a mountain by itself, and in its isolation, in 

 its accessibility, in its simplicity of structure, and in its relation to the local system of the 

 plateaus, it offers a richer harvest to the geologist, who shall give it a thorough study, than any 

 other single mountain with which I am acquainted " (563) . The removal of the weaker covering 

 strata, the topographic effects of which will be further considered in a later section, has laid 

 bare the harder core: 



"The rapid destruction of the lower Trias shales, and the stubborn resistance of the underlying [Carbonifer- 

 ous] limestone, have led to the baring of a broad area of the upper surface of the limestone, and this great exposure 

 of a single stratum reveals some details of structure that could not otherwise be comprehended without laborious 

 study. The rock appears to be divided into blocks of such magnitude that their superficial areas would be 

 expressed in miles rather than in acres, and these blocks have been inclined with somewhat different dips and 

 directions, so that at their edge they differ in altitude. So far as my limited observation goes they are not 

 separated by faults, but are connected by monoclinal flexures ... of small throw, and separated by wide 

 intervals. They are not parallel, but bear toward all points of the compass and intersect each other . . . 

 Their study cannot fail to throw light on the function of rigidity as a factor of orographic corrugation" (586). 



The Zuni dome, which was not seen until the third season of field work, gave support to 

 the view that Gilbert had previously reached in his examination of the basin ranges and of the 

 faulted plateaus ; for it was concluded that although, as a remote cause of the domelike upheaval, 

 "there may have been horizontal motion of subterranean matter, the immediate cause could 

 only be an upward motion" (564); and this goes well with the conclusion announced in the 

 chapter on the basin ranges in the report on the first and second field seasons: 



The movements of the strata by which the ridges have been produced have been in chief part vertical 

 along planes of fracture, and have not involved horizontal compression (42). 



Gilbert's tendency toward quantitative methods in geology, which like much of his careful 

 reasoning may be regarded as an outcome of his capacity and training in mathematics, is illus- 

 trated not only in the above estimate of the volume of the Zuni upheaval, but in another aspect 

 of the same problem. He wrote, regarding the deformation of the dome: 



If, as is probable, the strata of the Carboniferous limestone are continuous across the Archaean near its 

 crest . . . then the absolute length of the curved strata can be measured and compared with the direct distance 

 between their remote parts; and there is reason to hope that, by a series of such measurements in different 

 parts of the range, an answer can be found to the question whether, in the production of the curve, the remote 

 portions of the strata were brought nearer, or whether the curved portions were stretched (566) . 



THE UPHEAVAL OF MOUNTAINS 



It may be pointed out that Gilbert's views about the upheaval of the Zufii dome were in 

 a certain sense reactionary, for they were announced at a time when, after long discussion, 

 vertical forces, which were the mainspring of geological movements in an earlier stage of the 

 science, had been largely replaced, especially with respect to mountain masses, by forces of 

 lateral compression. While his views concerning the vertical upheaval of fault blocks in the 

 plateau province did not encounter opposition, it is probable that the outspoken dissent from 

 his explanation of the basin ranges also by vertical forces was favored by the geological theories 

 most in fashion at the time his reports came out. 



Reactionary as Gilbert's views in this matter were, they were less revolutionary than a 

 later explanation for high-standing plateaus advanced by Suess, to the effect that such plateaus 

 stand high because the surrounding lands, once at similarly lofty levels, had later subsided. 

 Had this explanation come to be accepted, Gilbert's views would have been wholly contra- 

 dicted; but as a matter of fact his reactionary views were prophetically correct, inasmuch as 

 they have come to prevail to-day not only for the plateau and the basin range provinces to 



