80 GROVE EARL GILBERT— DAVIS ^^'^vouxxt 



ever, is not in the mountain itself, but depends on the association of the phenomena of curvature and dike and 

 sheet with laccolites, in other mountains of the same group (27). 



The importance of progressive erosion in developing the mountain is alluded to again on 

 a later page, where after explaining that the laccoliths of the higher zone are already well exposed 

 in strong relief by the degradation of the surrounding strata, while those of the lower zone are not 

 yet so prominent as they will be later, Gilbert gracefully added : 



In attaching to the least of the peaks the name of my friend Mr. Holmes, I am confident that I commemo- 

 rate his attainment by a monument which will be more conspicuous to future generations and races than it is to 

 the present (150). 



RECOGNITION OF LACCOLITHS 



It is interesting to learn from the field notebooks of Gilbert's first season in the Henry 

 Mountains that their true interpretation was very early forced upon his attention. He 

 approached their southern members from the west, and on August 18, 1875, was on the great 

 Water-pocket flexure near the point where Hoxie Creek, about 12 miles north of its junction 

 with the Colorado, departs from the weaker strata of the flexure and cuts a singular horseshoe 

 canyon in the harder beds of the western rise. He then noted, concerning the southernmost 

 of the mountains, distant nearly 20 miles: "Ellsworth shows no volcanic colors but looks as 

 though built of the valley rocks. In the region a [the summit part of an adjoined sketch] I 

 can make out no dip but in the regions b & c [the southern and northern flanks] I measure dips 

 of about 25°." The next day while still on the same great flexure he wrote: "I see Ellsworth 

 better. On this [western] flank the dip is this way unmistakably. I can see the successive 

 outcrops circling around it — red at base, then white — & the white probably caps the s ummi t." 

 Thus the doming of the strata over the mountain top was detected. 



Gilbert and three of his party "slept out" that night, as explained the morning after: 

 " Darkness overtook us & we barely made a water pocket in the descent when we were forced 

 by the uncertainty of the way and by weariness to stop. At 4:15 this morning we started 

 again & reached breakfast at 6:30." It was then briefly noted regarding two mountains north 

 of Ellsworth: "Pennell and Hillers still look very volcanic"; this probably meant that their 

 igneous rocks were better revealed than those of Ellsworth, not that they were of eruptive 

 origin. Later in the day the basal features of Ellsworth were seen to be repeated in Hillers: 

 "The hogbacks seem to trend in a curve around the mountain flank as far as they extend"; 

 and the following day it was added: "The bending or swinging of the hogbacks about the 

 mountain base is unmistakable." A part of the mountain next north of Ellsworth, called 

 "Henry V" in the notes and later named Mount Holmes, was in view the next day, August 22: 

 "It was just a tumor, cracked in the middle," the cracks being filled with dikes; and to this 

 was added: "I am impressed with the idea that the dikes are radial, diminishing outwards. " 

 The next day. the same mountain was described as of "bubble form . . . the strata being 

 nearly level on top & the crests controlled by dikes, which are radial." Later on the same day 

 the same mass was compared with others: "It is a low-angled bubble. Hillers is high-angled 

 & Ellsworth strikes a mean." A significant generalization had already been recorded the day 

 before regarding Hillers: "Only the Trias was lifted & the Carboniferous either lay below the 

 seat of action or below a distributing reservoir"; thus the idea of a reservoir or cistern of igneous 

 rocks on an undisturbed foundation is first announced. The idea was at that time presumably 

 based more on the manifest arching of the Triassic strata, which were lifted but not intruded 

 by the igneous mass, than on any observed occurrence of Carboniferous beds beneath the moun- 

 tain here examined. Indeed, the horizontal underlying strata were imperfectly noted during 

 the brief visit to the mountains in the summer of 1875, yet a small figure, here copied, is added 

 in which the undisturbed underlying strata are clearly represented. Not until the following 

 s umm er were the trachyte reservoirs given a special name, "Lacune"; and this was modified in 

 the published report to "Laccolite," in which the root has the Greek rather than the Latin form. 



The mental processes of description, comparison, and generalization can find no better 

 illustration than is offered in the first 50 pages of the Henry Mountains report; but it is well 

 to recognize at the same time that these 50 pages also afford an admirable example of the 



