Mat 7, 1909] 



SCIENCE 



753 



sion are so impressive on every hand, while the 

 recognized absence of an abundance of running 

 waters makes it appear that the progress of ero- 

 sion must be extremely slow. 



In southwestern United States, where the lofty 

 and numerous desert ranges have been regarded 

 as having developed out of an old peneplain, a 

 remnantal portion of which seems to be repre- 

 sented in the great Mesa de Maya, where through 

 faulting and folding there has been produced a 

 rapid succession of resistant and weak rock-belts, 

 where deflative phenomena are thought to be 

 typically expressed, and where there are several 

 large rivers flowing from the humid zones of the 

 higher Eockiea in deep canyons through to the 

 sea, there appears to be all of the data at hand 

 by which to measure not only the relative rapidity 

 of deflation upon contiguous areas of hard and 

 soft rocks, but also to gauge the comparative 

 effects between erosion by direct deflation and 

 erosion by corrasion xinaided by extended chemical 

 decomposition of the rocks at the surface. 



Locus of Maximum Lateral Deflation in Desert 



Ranges: Chaeies E.. Ketes. 



Deflative erosion in an arid land is preeminently 

 plains-forming. Its general effects are best likened 

 to the work of the sea along an exposed coast 

 where there is carved out of the shore a marine 

 platform. In a smaller way, as a detritus-laden 

 stream impinges against its banks, forming high 

 bluffs or cliflFs, so in the dry regions the swiftly 

 moving air-current heavily charged near the 

 ground with sands and dusts tends to wear away 

 fastest the least sheltered portions of the desert 

 hills and mountains. 



The air-currents of the desert are both strong 

 and constant. Their transportative powers have 

 never been measured quantitatively. When trans- 

 portation is active the " sandstorm " results. The 

 personal discomfort to the traveler in a sand- 

 storm is so very great that he is usually oblivious 

 to all else. The volume of soil flowing along the 

 surface of the ground during one of these storms 

 must be prodigious. Compared with the amount 

 of sediments carried along by some large river, as 

 the Mississippi in time of flood, it is estimated 

 that in the lower twenty feet of the deflation- 

 stream there are equal amounts of rock-waste 

 moving in like cross-sections of the great river 

 and of the air-stream of the desert. The air- 

 stream moves forty miles an hour instead of four, 

 as in the case of the water-stream; and in place 

 of being only a mile wide the path of the sand- 

 storm is several hundreds of miles in width. The 



lower six inches of the air-stream is almost wholly 

 moving sand and fine gravel. The finer dust soars 

 upwards thousands of feet, darkening the light of 

 the sun as by a heavy thunder-cloud. 



Over surfaces of drifted sands and of weak 

 rocks the erosion is mainly accomplished by a 

 trituration of the particles of the heavily sand- 

 laden bottom stratum of moving air. Whenever 

 bare or hard rock-masses are encountered there- 

 is vigorous sand-blast action. Thus, hypsometric- 

 ally, among the desert ranges which are all com- 

 posed of very hard rocks usually devoid of a 

 soil-mantle, the notably exposed zone is at the. 

 very base of the mountains, or immediately above 

 the surface level of the surrounding plains. This 

 is probably the chief reason why there are no foot- 

 hills flanking the mountain ranges of the desert, 

 why mountain and plain so sharply meet, and 

 why the bases of the desert ranges are often so. 

 abrupt and straight as to suggest at once the 

 presence of fault-scarps as an explanation of the 

 steep faces to the mountains. 



When we institute search for direct evidences of 

 fault-lines which are supposed to give rise to the- 

 escarpments, we usually look in vain. Although- 

 the mountain may be a faulted block the move- 

 ment, however profound, is commonly discovered 

 to be of ancient date. Its fault-plane is found to 

 be far out in the plain, often at distances of four 

 or five miles. The intervening space has a smooth- 

 and gently sloping rock-floor and has every ap- 

 pearance of a marine plain of denudation from, 

 which the sea has but recently retired. McGee' 

 notes many such plains with extensive rock-floors 

 fashioned from the hardest rocks, among the 

 desert ranges of Sonora in Mexico. Others are 

 described more in detail in the New Mexican, 

 region.' They are now widely known throughout 

 the arid lands of the west. Thus, in the general 

 leveling and lowering of the desert region an- 

 ciently faulted and planed off so as to present 

 alternating belts of resistant and weak rocks the 

 areas of the latter are worn chiefly downward by 

 the wind-action, but the hard mountain belts 

 which have emerged from the softer areas are- 

 attacked laterally; and the zone of maximum 

 eolation is at and just above the general plains- 

 level. 



A New Trachodon from the Laramiie Beds of Con- 

 verse County, Wyo.: Chables H. Stebnbeeq. 

 A complete skeleton, except hind feet, one tibia- 

 and fibulEB and tail vertebrse. It lies on its back. 



'Bull. Oeol. 8oc. America, VIII., p. 87, 1897. 

 ■ Hid., XIX., p. 78, 1908. 



