976 



SCIENCE 



[N. S. Vol. XXV. No. 651 



and currents of the ocean during submergence 

 of the land. 



But these views of the subaqueous distribu- 

 tion of the detritus have not found favor and 

 are not accepted. The consensus of geologic 

 opinion appears to be that the slopes are 

 wholly subaerial in origin; the result of 

 stream action and distribution alone, without 

 oceanic aid. Their continuity is explained by 

 Gilbert and others, by their contiguity and the 

 coalescence of many adjoining alluvial cones, 

 each with its apex at the mouth of a gorge or a 

 canyon, producing about the mouth of such 

 gorge a symmetric heap of alluvium. Gilbert 

 writes: 



Rarely these cones stand so far apart as to be 

 completely individual and distinct, but usually the 

 parent gorges are so thickly set along the moun- 

 tain front that the cones are more or less united, 

 and give to the contour of the mountain base a 

 scalloped outline. 



This view was accepted by Eussell, who 

 quotes it in his paper, in the Geological 

 Magazine.' 



It is accepted also, by Geikie, Dana and 

 others. 



In geological literature the slopes are 

 usually described as alluvial fans, alluvial 

 cones, or talus fans. We are indebted for 

 most of these terms to Dr. Drew, who, de- 

 scribing the alluvial and lacustrine deposits 

 and glacial records of the Upper Indus Basin, 

 uses the terms ' fan talus ' and ' alluvial fans ' 

 for the detrital accumulations at the mouths 

 of canyons. In the illustrations these ac- 

 cumulations of detrital material are repre- 

 sented as local and very steep in inclination, 

 with radii perhaps a mile in length. The 

 ' amalgamation ' or union of fans is recog- 

 nized, and examples are given of the denuda- 

 tion of fan slopes.^ 



Dr. Ida H. Ogilvie, describing the topog- 

 raphy of the Ortiz Laccolith in New Mexico, 

 proposes the name ' conoplain ' because of the 

 outward slope of the surface in all directions. 



- Geological Magazine, N. S., Decade III., Vol. 

 VI., No. VII., July, 1889, p. 290. 



' Frederic Drew, Esq., LL.D., F.G.S., Quarterly 

 Journal, Geological Society of London, 1873, p. 

 441. 



This sloping' surface is regarded as the result 

 of subaerial stream action, under substantially 

 present conditions, and it is suggested that the 

 broad areas of the great plateaus have been 

 produced by a similar process. 



Evidences of Submergence.- — The great 

 dominant fact showing that the slopes in their 

 original integrity of form were deposited and 

 shaped during submergence, and that they 

 were molded by oceanic currents, rather than 

 by the waters such as now exist, is that the 

 slopes, or the portions to which I refer, are 

 older than the streams. They antedate the 

 streams and the valleys of the existing drain- 

 age. The conditions under which the con- 

 tinuous higher slopes were formed are 

 obviously very different from those now exist- 

 ing. The action of the streams of the present 

 is one of redistribution of the material of 

 the slopes. It is destructive of the higher 

 and older slopes, and not upbuilding or con- 

 structive except at lower levels of deposition. 

 It is true that the materials of portions of the 

 slope are carried downwards and onwards and 

 are deposited at lower levels, where the volume 

 of water is diminished by wider distribution 

 and by absorption. We thus have what may 

 be called primary and secondary slopes or, 

 if preferable, initial and derived sloi)es. The 

 secondary slopes, now forming, are in washes 

 and arroyos below the surface of the primary 

 slopes, remnants of which are left on each 

 side sufficient to preserve the grand topo- 

 graphic features and scenic effects far above 

 the reach of modern floods. The higher parts 

 of the flanking slopes do not to-day receive 

 accessions from the canyons, they are not 

 being built higher and are not the product of 

 modern streams. The conditions of deposi- 

 tion were evidently very different from those 

 of to-day. The phenomena all point to the 

 formation of the primary slopes during a 

 period of subsidence before the great Post- 

 Miocene continental uplift. The amplitude 

 of this movement was doubtless greater than 

 is indicated by the height of the initial slopes. 



Generally throughout the southwest, the 

 upper margin of the slopes is between the 



^American Geologist, Vol. XXXVl., July, 1905. 



