June 21, 1907] 



SCIENCE 



975 



the impression that the wide ocean must have 

 been the agent causing such sweeping outlines 

 and regularly inclined surfaces. 



The great table lands, or plateaus, so-called, 

 of the Great Basin region are, in fact, a series 

 of long slopes and not tabular surfaces. This 

 is most notable in the southern portions of the 

 basin, where the general surface is inclined 

 from an altitude of about 4,000 feet downward 

 to the sea-level, and even below that level, as 

 at Death Valley, Nevada. The bordering 

 mountains, and each detached mountain ridge 

 or peak in that region, known as ' Lost Moun- 

 tains,' are flanked by detrital accumulations 

 from the high rocky ridges. 



A fine example of a flanking slope is found 

 on the north side of San Bernardino Moun- 

 tain, California, extending from the summit 

 of the Cajon Pass to the Mohave River, a dis- 

 tance of 19 miles, with a grade, or ascent, of 

 about 105 feet per mile. It is traversed by 

 the old Spanish Trail and seems like a plain 

 to the traveler when riding in a Pullman car. 



In the region of Tucson, Arizona, there 

 are fine examples of long slopes of great regu- 

 larity flanking the Santa Catalina Mountains, 

 the Santa Ritas and the Sierritas. 



The length downward of the Santa Cata- 

 lina slope on the south side, measured from 

 the 3,500-foot contour down to the Rillito, or 

 to the 2,500-foot contour, is five miles, and the 

 difference of elevation is 1,000 feet, giving a 

 fall of 200 feet to the mile. This slope may 

 be regarded as the upper portion of the former 

 slope, the lower portion having been swept 

 away by the Rillito and Santa Cruz drainage. 

 If, as we may believe, the slope originally 

 extended as far as the Santa Cruz River, or 

 ten miles in length, the average fall would be 

 100 feet per mile, thus agreeing with the 

 grade at the Sierritas Slope, and at the Santa 

 Rita western slope. 



This western flanking slope of the Santa 

 Rita mountains is a good example of the regu- 

 larity of descent, and of continuity. From 

 the 4,000-foot contour near McCleary's camp 

 down to the 3,000-foot contour above the 

 Santa Cruz river, the distance is eight miles, 

 and the fall is 1,000 feet, or about 125 feet to 

 the mile; but if the lower portion of this 



slope had not been cut away by the river the 

 length would be ten miles, and the average 

 grade 100 feet to the mile. 



The slope of the Sierritas range, southeast 

 of Tucson, is remarkable for its average grade 

 of about 100 feet to the mile for the distance 

 of ten miles. The grade increases nearer the 

 mountains in this as in other examples. 



The north side slope of the same range is 

 equally or more remarkable for its extent and 

 even grade. 



The valley of the Sonoita from Calabasaa 

 eastwards to the summit affords fine examples 

 of the ancient slopes and their deep erosion 

 by the river, leaving terraced banks. 



OHgin of Detrital Slopes. — The form and 

 origin of detrital slopes, especially those of 

 the southern portion of the Great Basin, are 

 discussed in my report of Explorations in 

 1853.' The broad general molding of the 

 slopes by oceanic action is there recognized as, 

 also, the present modifying action of streams 

 and flood-waters, which are described as 

 spreading out over the lower portions of the 

 slopes ' fan-like ' in a multitude of channels, 

 but at lower levels than the upper portions of 

 the slopes which are cut through and left high 

 above the existing stream. Such slopes are 

 in strong contrast with the localized phe- 

 nomena of modern deltaic detrital deposits at 

 the mouths of mountain gorges or canyons, 

 known as ' alluvial fans ' or ' alluvial 

 cones.' While doubtless such delta-like de- 

 posits, and the long slopes, have ini- 

 tially the same origin in this respect, at 

 least, that all are made up of debris from 

 the high rocky ridges; the flanking slopes to 

 which I wish to direct attention are less 

 localized, are without the deltaic form, are 

 much more extensive and broader,, and are 

 generally without evident relation to any par- 

 ticular canyon or gorge of the mountain, and 

 do not exhibit the scalloped outline of inter- 

 section claimed for alluvial fans. They show ' 

 the operation of a widely extended distributing 

 or leveling agency which it would appear 

 could not have been other than tides, waves 



' ' Report of a Geological Keconnaissance in 

 California,' 1855, pp. 214-217. 



