46 GROVE KARL GILBERT— DAVIS [MEUOlRs ivt™xi, 



the Mount Taylor Plateau, is a steep-sided butte, which was recognized to be " the flue through 

 which an eruption reached the surface. ... It is a cast in lava, of which the mold was the 

 conduit of a volcano, now not only extinct but demolished. . . . The Cabezon and other 

 similar pinnacles on the opposite [northern] side of the Taylor plateau" were similarly explained 

 (534-536) . At a point where the Gila River has eroded a deep valley in lava beds, a view of 

 one valley wall from the cliff top of the other revealed a cinder cone, 600 or S00 feet high, rest- 

 ing on older flows and completely buried under later outpourings (538) . 



A delicate item of truly Gilbertian quality, an "illustration of the principle of rhythm 

 in nature," remains to be mentioned. It is the occurrence of " wave-like heaps " of basalt frag- 

 ments on the long and gentle slope beneath many mesa cliffs ; each heap represents a large slab 

 of lava that has been detached from the cliff by undermining and has then slowly settled, 

 "without notable horizontal slutting, as the subjacent material is eaten away by percolating 

 waters. " When the heaps are of large size, a little swale with moister soil occurs behind each 

 of them; the swales are sometimes cultivated by the Indians. As the size of the slabs, when 

 they break off after undermining has progressed sufficiently, is fairly constant for a given sheet 

 of lava, there arises " a rhythmic uniformity of result, as nearly perfect, perhaps, as that of the 

 analogous waves of the sea" (537). 



VAEIOUS MINOR TOPICS 



The richness of the western field is well known. It incited Gilbert to discuss, briefly, it 

 is true, various topics that lay somewhat aside from his main lines of study. The arid areas 

 afforded abundant illustrations of "the efficiency of dry sand as an erosive agent, when borne 

 by the wind," as had already been noted by earlier explorers, and Gdbert was thus led "to 

 attach considerable importance to this agent of terrestrial denudation. . . . Such wearing 

 cuts no canons, and leaves no grand monuments of the magnitude of its results, but it is never- 

 theless a true denudation, applied to broad areas, and, where water is deficient, is no incon- 

 siderable factor in the sculpture of the land" (83). Two instances of such scuplture may be 

 here referred to because they both concern localities where other significant matters curiously 

 enough received no consideration. In one instance attention is called to the conspicuous 

 irregularity of the fanciful and grotesque forms into which certain cross-bedded cliff-making 

 sandstones are wind-carved; but, as noted above, nothing is said either there or elsewhere about 

 the possible origin of the cross-bedding in the sandstone by ancient aggradational wind action. 

 In the other instance an account is given of the action of the wind in removing weak shales 

 from beneath the Triassic sandstones of the Vermilion Cliffs, west of the junction of Paria Creek 

 with the Colorado at Lees Ferry; but although this district is described in some detail in field 

 notebooks, and is mentioned repeatedly in the first report (52, 67, 83, 84), nothing is said of the 

 huge and disorderly landslides that are strewn for miles along the cliff base; but Powell and 

 Dutton were equally inattentive to these tumultuous downfalls, perhaps because they are only 

 subordinate details among the colossal cliffs, platforms, and canyons of this large-featured 

 region. 



A more explicit treatment of some phases of wind erosion was given at the summer meeting 

 of the American Association for the Advancement of Science, at Hartford, in 1874, which 

 Gilbert had the opportunity of attending because he did not go west that year and also because 

 he was then attracted into New England for personal reasons. Although his paper was pub- 

 lished only as an abstract, with a footnote intimating that its substance would " appear officially 

 and more fully" in the Wheeler report, this intimation is not borne out in the pages of Volume 

 III. Wind-borne sand was described as — 



a denuding agent worthy to be mentioned in the list with frost, and flood, and wave. ... It undermines 

 cliffs; it scours mountain passes; and it reduces open plains. . . . The degradation of plains by the wind cannot 

 be measured, because it leaves no such monuments as does denudation by water. Water is a leveller in the sense 

 that it transfers material from higher places to lower; but, where it erodes, it . . . leaves ridges and islands, by 

 which its results can be measured. The wind, on the contrary, works most diligently upon salients, and strives 

 to smooth away every vestige of the surface it remodels. 



