academy op saENCEs] WHEELER SURVEY 45 



the Santa Fe Railway — as it indeed had been their regret near the beginning of the excursion 

 at Niagara and near its middle in the Great Basin — that, on account of ill health, Gilbert could 

 not be of the party and tell to the European guests something of his early work in the Far West. 



VOLCANIC FEATURES 



Features of volcanic origin, already known in part from the reports of earlier observers, 

 were found by Gilbert in abundance and variety and given illuminating description, although 

 as his later work led him away from volcanic problems, his name is not usually associated 

 with studies of this kind. The young cinder cones of the Sevier Desert have already been 

 mentioned ; they are practically unchanged by erosion. " The weathering of the frail scoria, 

 that caps the crater rim, does not seem to have been begun; the taffylike pellets that, spattered 

 from the bubbling caldron, fell half cooled upon its wall seem as though congealed, but yester- 

 day" (136). In the surrounding lava beds, the "most interesting feature is the existence of 

 a number of caves, produced by the escape of lavas from their channels, after the formation of 

 a self-sustaining crust. The caves he entirely below the general level of the lava field, and we 

 discovered them only where portions of their roofs had fallen." The tubular aperture of one 

 cave was followed "for one or two hundred feet. . . . The width . . . averaged 30 feet and 

 the depth 18, and in length it extended indefinitely beyond the section we explored" (141). 



Mount San Francisco, on the plateau south of the Colorado Canyon and perhaps the 

 youngest of the larger volcanoes, was ascended in the summer of 1871 and 65 cinder cones 

 were counted from its summit. Farther southeast, in the Mogollon area, a rich variety of vol- 

 canic features was found in Sierra Blanca, composed of massive trachyte of imperfectly conical 

 form, nearly 3,000 feet in local height, with — 



a remarkably low angle of slope . . . long slopes of sanidin-dolerite, that appear to have flowed from side fis- 

 sures . . . spread in successive sheets over the plain ... to the east . . . for ten or fifteen miles, and to the 

 west for thirty miles. . . . Scattered over these broad sheets are rounded cinder cones, not exceeding a few 

 hundred feet in height, and with some of them are associated coulees of balsalt. The depth of the water-worn 

 gorges upon the flanks of Sierra Blanca, attest the antiquity of its chief mass, and in some of these gorges have 

 run streams of basalt. In the valley of White Mountain river . . . are vestiges of three distinct lava flows, 

 which entered at as many different epochs in the progress of the excavation of the valley, and have been 

 successively cut through by the stream (527). 



Lava sheets of various dimensions were seen in all stages of preservation and erosion. 

 Some of the most recent examples were found near the southern rim of the plateaus, where 

 " the flowing lavas have in part overrun the cliff and poured into the valleys of the Verde and 

 its tributaries. The principal roads connecting the upper and lower countries avoid the preci- 

 pice ... by following the easy grades of these black congealed rivers" (130). Another 

 recent flow lies in the valley of the San Jose, a western branch of the Rio Grande; many trav- 

 elers on the Santa Fe Railway, which now follows this valley, will agree that " the convolutions 

 of the viscous current, presented as perfectly as though cooled but yesterday, afford there a 

 wonderful and impressive spectacle" (533). Another flow of greater age followed the valley 

 of Zuni River, a branch of the Little Colorado, for 50 miles, where it crosses the New Mexico- 

 Arizona boundary, and caused "a curious duplication of the valley, which has been deepened 

 by later erosion on both sides of the lava" (533). Not until the third season of exploration 

 was it known that "within the borders of Arizona and New Mexico lies one of the great lava 

 tracts of the world, second in magnitude in our country only to the great northwestern lava 

 field, and fifteen times as large as the classical district of extinct volcanoes in Central France" 

 (525) ; this mention of a small European prototype of a large American feature being one of 

 the few of its kind in Gilbert's reports. 



A number of lava-capped plateaus and mesas are described, among the larger ones being 

 those of Mount Taylor and Acoma (534, 554), the latter measuring 30 by 15 miles. Two ex- 

 amples in far-advanced stages of erosion are illustrated by views and sections. One of these, 

 in the valley of the San Jose, is a "lava cone on a pedestal of sandstone and shale. . . . What- 

 ever sheet of basalt surrounded the cone has been undermined and destroyed up to its very 

 base, where the increasing thickness of the cover has retarded the work." The other, near 



