GEOLOGIC EVIDENCE. 315 



Lower and Upper Carbonic, and early Permic. The eastern Triassic beds, and 

 those of the Rocky Mountains, are nearly everywhere red throughout, and 

 there is considerable red color in the Lower Cretacic of the Great Plains area. 

 Then, too, there are many red beds in the Proterozoic of America as well as 

 of Europe. Between these zones of brilliant strata are the far more widely 

 distributed ones of grays and darker colors, and these are deposits of the times 

 when oceans have most widely transgressed the lands, and therefore the times 

 of greater humidity. The maximum of continental extension falls in with red 

 deposits and more or less arid climates." 



The close association of red beds with gypsum and salt deposits in the 

 Triassic and Permian, together with the occurrence of red beds in practically 

 all the periods in which salt and gypsum were deposited, strengthens the corre- 

 lation of each with aridity. A salt-bed, but especially one of gypsum, implies 

 a constantly arid climate during the period of deposition. Red beds, according 

 to BarreU, are formed during a period which is hot and alternately wet and 

 dry. This difference in the conditions of their deposition seems to be signifi- 

 cant of their association. A region of great aridity where salt and gypsum are 

 being deposited would almost certainly be surrounded by subarid transition 

 to humid regions, in which short cycles of wet and dry years would afford the 

 conditions most favorable to oxidation. Moreover, the appearance and dis- 

 appearance of an arid phase in a humid climate would probably be marked 

 by a period of pidsation, i. e., of alternating wet and dry conditions. Such a 

 sequence of facts seems to be indicated by the Permian beds of Kansas, which 

 are overlaid by beds of gypsum and salt, and these by red beds, which in tiu-n 

 are followed by darker humid deposits. Even more suggestive is the occur- 

 rence of gypsum and salt in the Triassic red beds of the eastern front of the 

 Rocky Mountains (plate 58 a). 



Terraces. — ^Huntington (1914 : 31) has recently advanced an interesting and, 

 to one familiar with arid regions, a compeUing explanation of terraces: 



"If moist conditions give place to aridity, many other changes wUl take 

 place. The forests and a large part of the other vegetation wiU die; the 

 streams will diminish in volume, many will dry up entirely part of the time, 

 and will fail to reach the main river except in occasional floods. The death of 

 the vegetation will lead to the denudation of the mountains, and ultimately 

 the slopes will become almost absolutely naked, as they are in Persia. The 

 rapid removal of soil from the slopes of the mountains will inevitably increase 

 the load of the streams, and in many cases wiU overload them. Accordingly, 

 wherever the grade is less steep than on the slopes or in the minor tributaries, 

 the advent of aridity wUl cause deposition to begin at once, either at the base 

 of the mountains or in the larger valleys. This process of deposition tends to 

 build up deep accumulations of gravel in the valley bottoms, and vast fans or 

 alluvial aprons (bahadas) at the base of the mountains. Thus, so long as 

 aridity continues, the main mountain vaUeys and the piedmont regions tend 

 to retain all the material which comes down from the mountain. 



"To complete the process of terracing the only requisite is a return to moist 

 conditions. Vegetation will increase in amount, the steams will become more 

 uniform in size from season to season, the gravel deposits will become saturated 

 with moisture, the water of the streams will be less subject to loss by sinking 

 into the ground and by evaporation, and the streams will become longer. In 

 their upper portions, they will be suppHed with waste less abundantly than 

 hitherto, because the greater abundance of vegetation will tend to hold in 



