74 GROVE KARL GILBERT— DAVIS ^^^vo^xiti, 



No other account of this natural leaning tower is known. Time was repeatedly taken for 

 records of mountain and valley breezes, descending at night and ascending by day, and Gilbert 

 later spoke on this subject before the Philosophical Society of Washington. A remarkable dis- 

 cussion of the relation between the diurnal range of air temperature and the "barometric 

 horary curve" was made on September 6; this evidently foreshadows a graphic discussion of the 

 same problem in an address 10 years later, as will be told in due season. East of the plateaus the 

 greatly denuded Water-pocket flexure — recorded in the notes under the name of "Escalante" — 

 is described as of unsymmetrical anticlinal form, with steep dips on the east and very gentle 

 dips on the west; its axis is slightly arched, so that the Vermilion sandstone, removed from the 

 highest part of the flexure, overrides it in strong cliffs at points 30 miles apart north and south 

 and these two crosswise cliffs are "connected by continuous (except for canons) escarpments 

 in the two slopes of the fold, the NE standing under the steep fold slope & the SW standing from 

 5 to 10 miles down the gentle fold slope. So there is a continuous cliff of erosion returning in 

 itself & facing inward — the reverse of a mesa. Inside the Vermilion the Shinarump circles in 

 the same way. I will call the upper of the cliffs 'The Circle Cliffs'." The phrase, "reverse 

 of a mesa," is certainly felicitous. 



The faults and flexures between the various members of the high plateaus were examined 

 with care, and the relative values of displacement and erosion in producing the existing relief 

 was especially studied. One of the most dehberate records, written "on a crag overlooking the 

 lake," July 8, 1875, relates to this problem: 



What was the origin of Fish Lake Valley? 1. Its walls are of trachyte. Are they massive eruptions over 

 dikes — hills of eruption? No, for they exhibit in their escarpments a bedded structure & their slopes are too 

 steep & too definitely angled at top ... 2. Is it the result of aqueous erosion? No, for it is too broad to 

 have been cut from the trachyte since the age of the trachyte, and its side canons are disproportionally 

 small ... 3. It is not the result of glacial erosion, for the glaciation of this region is but slight — entirely 

 inadequate to the making or shaping of a great valley. 4th. It remains to suppose that since the trachyte epoch 

 the floor of the valley has sunk (or the adjacent hills have risen) an amount equal to the present depth of the 

 valley, plus the depth of its detrital filling, plus the loss of the uplands by waste* — an amount somewhere between 

 2000 & 3000 ft. 



There was no "jumping at conclusions" in a study thus conducted. 



TWO VISITS TO THE HENRY MOUNTAINS 



The most important observations made by Gilbert during his first season of field work 

 under Powell concerned the Henry Mountains, which rise in an arid and relatively inaccessible 

 region east of the Water-pocket flexure and north of the Colorado River. They were examined 

 in the fortnight beginning August 19, 1875, and the structures then discovered proved to be of 

 so great interest that the mountains were made the subject of a special investigation in the cam- 

 paign of 1876, when two months of a field season that extended from early August to late 

 November were devoted to them. Thus originated one of Gilbert's most famous studies. The 

 mountains are conspicuous objects in the eastward prospect from the rim of the high plateaus, 

 from which they are 30 miles or more distant ; and it is probable that their striking forms rising 

 in the distance excited Gilbert's wish to look at them more closely. On the way across the 

 intermediate barren country he had a distant sight of Navajo Mountain, south of the Colorado 

 Canyon, and discovered that, like the Henry Mountains, it also has upturned strata around its 

 base: 



On all its visible flanks, from NE around by N & W to SW, [it] is built of Trias rocks dipping away from 

 its center. It is truly a volcanic cone of elevation. ... I measure 11° at the NE & 15° at the SW on the 

 flanks — dips that would carry the Trias almost to its crest, but its crest is dark with lava. 



The novel structure of the Henry Mountains appears to have been recognized in the fort- 

 night of their first examination, and the problem of their origin was then in part formulated; 

 for on September 5, a question is recorded : 



By the way, how do the flexures of Ellsworth and Henry V [members of the group] & Navajo consist with 

 the violence belonging to irruption & eruption. Can deeply buried strata be bent without the slowness of action 

 necessary at the surface ? 



