WILLIS] GENERAL GEOLOGIC NOTES 19 



cies, among which wind has been the dominant one, having worked 

 the material into a uniform condition. 



Alhivial deposits, composed of unmodified river mud, such as 

 rivers cany and deposit in regions of abundant rainfall and vegetation 

 consist of coarse as well as of fine clays and sands. They contain 

 also more or less carbonaceous matter derived from vegetation. But 

 alluvium, which is spread and dries on flats that for any reason are 

 not covered by vegetation, is sorted by winds, the fine clay being 

 blown out and the coarse stuff being left behind. By being blown 

 about or against one another the sand grains are worn down. There 

 is also chemical disintegration. By many repetitions the process 

 results in a fine flour of the most enduring universal substances, 

 aluminous clay, silicious sand, and oxide of iron. Of such is the 

 Pampean terrane. 



Thus the Pampean, in the physical and chemical constitution of its 

 materials, is a product of processes which require the interaction of 

 rivers and winds. On the Arctic plains of northern Siberia, where 

 the great rivers flood vast areas and retreating leave them bare, or in 

 the immense delta of the Hwang River in eastern China, we may 

 find modern illustrations of the I^ampean conditions. 



Climate is a factor of the first importance in modifying the efl'ect of 

 wind on alluvial deposits. Wind can not raise dust from surfaces 

 that are frozen, moist, or sufficiently covered with vegetation, and it 

 does not erode them. If alluvial deposits are dry and bare, wind does 

 erode even plane surfaces, and when confined by the configuration of 

 the surface to a hollow, or channel, it erodes rapidly. That the material 

 of the Pampean has been blown about and sorted by wind is clearly 

 proved by its uniform fineness, and hence we might conclude that the 

 pampas have been arid and bare. But the Pampean is in large part 

 a river deposit, such as the Hwang River has spread over the vast 

 delta plain of China, and has been distributed by floods in a similar 

 delta. The terrane contains abundant remains of large herbivorous 

 animals, which lived on grass that must have grown rankly and in 

 profusion. Thus there must have been rainfall sufficient to nourish 

 vegetation. These evidences of aridity and humidity appear to be 

 contradictory, but they are readily explained by geographic relations 

 and by changes of climate. 



The geographic relations of the Hwang River are to the point. 

 The river rises in the mountains of central Asia, flows through desert 

 basins, and descends to the head of its delta heavily laden with desert 

 dust. The delta plain which is built of that dust is comparable in 

 extent with the pampas of South America. The rivers that now 

 flow from the Cordillera of Bolivia and Argentina southeastward also 

 cross desert basins, which are arid because the mountains take the 

 moisture from the air currents. They have been more or less arid 



