ACADEMY OF SCIENCES] POWELL'S SURVEY 75 



It will appear in the sequel that the answer to this question was most ingeniously worked 

 out in the summer of 1876, when the facts of form and structure were carefully ascertained. 

 In both years Gilbert had the services of W. II. Graves as topographer. The five notebooks of 

 that second summer contain many sketches of the mountains, and it is evident from the numera- 

 tion of prominent points and from comments concerning them that Gilbert also contributed 

 largely to the work of topographic mapping. Indeed from Powell's report on the work of his 

 survey for 1876, one might suppose that Gilbert was not accompanied by a topographer, but 

 did all the work himself: 



A topographic survey of the Henry mountains was made [by Gilbert's party] in 1875, and a map constructed 

 on a scale of 4 miles to the inch; but this being thought too small a scale to admit of correct representation of 

 the details of the geology, Mr. Gilbert in addition to his geological work made [in 1876] a more detailed survey 

 of the topography, carrying on a complete system of secondary triangulation and a connected plane table sketch 

 over more than 1,000 square miles. The data collected are sufficient to make a topographic map of the Henry 

 mountains on a scale of 2 miles to an inch. 



In any case Gilbert was much interested in the topographic work. On October 12, a 

 rainy day spent in camp near the northernmost member of the mountain group, two pages were 

 given to a calculation of "spherical excess," in order to determine whether allowance should be 

 made for it in local triangulation; the result showed that "the excess of the whole polygon of 

 triangulation is not greater than 5" and is quite inconsiderable as compared with the probable 

 error of measurement of one angle, 1 minute." The many-sided competence which such an 

 entry illustrates was characteristic of all Gilbert's work. 



The novelty of the structural features discovered 

 in the Henry Mountains was so great as to compel close 

 scrutiny before they were accepted as definitely de- 

 termined even by their observer. It was indeed so 

 great as to awaken doubts in the minds of distant 

 readers as to the competence of a little-known geologist, 



as Gilbert then was, who should declare that rising Fi S- e.-Ideal section of a laccolith; from Gilbert's note- 

 lavas could bend upward the strata into which they rose. 



Such an explanation recalled the discredited theory of "craters of elevation," which after 

 strong advocacy by eminent geologists of an earlier generation had been gradually disproved 

 and abandoned by the leading geologists of Gilbert's time. It was like a backward step to 

 return to any such idea. But there was this difference: " Craters of elevation" were superficial 

 phenomena; and the upturned strata of the Henry Mountains, although now visible at the 

 surface, had become so only as the result of an enormous denudation; the upturning was a 

 deep-seated phenomenon. Whatever uncertainty others may have felt on this point, Gilbert 

 really proved his case beyond all possible doubt. Yet in spite of the large theoretical impor- 

 tance of the structures thus made known, their region is so remote that, since Gilbert's visit, 

 only one other geologist has given close study to this singular mountain group, and his report 

 upon it is not yet published. So remote indeed is the district of the Henry Mountains from 

 lines of ordinary travel that it is known in the surrounding region as a safety zone for those who 

 wish to "avoid doing business with the sheriff." During the Great War, certain persons who 

 were under suspicion of the Military Intelligence Division of the Army chose this very district 

 as a refuge because of its inaccessibility and lonesomeness. After they had reached its seclusion 

 they were visited by an officer disguised in his usual occupation of geologist, and so completely 

 did their isolation render them harmless that they were let alone in their voluntary internment. 



THE HENRY MOUNTAINS REPORT 



The problem of the Henry Mountains must have been truly of an inspiring nature, for the 

 report upon it was completed March 1, 1877, four months after Gilbert's return from the second 

 season in the field. Its date of publication on the title-page is the same year, although by reason 

 of delay in preparing certain plates, the report was not completed and distributed till 1879; 

 a second edition was issued in 1880. Short as was the time given to its preparation, it is one 



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