REVIEWS 735 



remnants of an old debris apron built in the latter half of Tertiary 

 time, and preserved because of lack of sufficient precipitation to pro- 

 duce run-off. The author takes the ground that this apron of waste 

 was deposited and arranged by streams, but shows that the streams 

 were not of the ordinary type. A careful analysis of arid region 

 stream work leads to the conclusion that "there is no such thing as 

 sheet flow" in the sense of a uniform flood of water and waste, but 

 that the more or less perfect sheet is "one of intimately lacing threads 

 of current." 



The deposits of which the plains are composed are the result of 

 this stream work and not of still water work. As proof of the proposi- 

 tion, the author finds the following facts : 



(i) Wide distribution, at all depths of gravels, which decrease in 

 size from the mountains. (2) The size of these coarse materials is 

 often that of cobbles. (3) Gravel courses run east and west and are 

 cross-bedded — marking the channel courses of eastward flowing 

 streams. (4) Sand bed salso occur in courses elongated east and west 

 in the "clays." (5) Interlaced stream beds occur as shown where 

 erosion has disclosed them. (6) Even most clay beds are thin and 

 elongated east and west. 



Mr. Johnson finds the "mortar-beds" cutting across the local 

 bedding of sands and gravels, and he considers the beds to have been 

 formed by the cementation of the coarse materials at the level of the 

 ground water by salts carried down in the sinking surface water. It 

 appears that his explanation of the distribution of water in these 

 gravels is similar to that of water distribution in the morainic materials 

 of Illinois and Indiana, as worked out by Leverett and others, and 

 stated by Leverett in the Eighteenth Annual Report of the U. S. Geol. 

 Surv. Pt. IV. 



Chapter III is a discussion of the "deficiencies of climate." The 

 climate of the High Plains is described as subhumid. It varies from 

 humid to arid through a period of years. The author very properly 

 states that the amount of precipitation is not the only factor in deter- 

 mining the climate, or even the arability of a region. The Dakota 

 wheat fields actually have less rainfall than the Staked Plains of Texas, 

 but the rainfall is more effective in Dakota. In Texas (i) rainfall is 

 more spasmodic; (2) temperature is higher, increasing evaporation 

 and decreasing relative humidity; (3) there are more hours of sun- 

 shine, and (4) greater wind movement. In the summary he reaches 



