1914] CURRENT LITERATURE 533 



by the same author 6 we had learned to know something of the fascination and 

 scientific interest of these dynamic landscapes, and now we have his detailed 

 results. 



The Nebraska sandhill country covers an area of about 18,000 square 



miles, that is, nearly one-fourth of the area of the state. There are similar 



but smaller areas of sandhills in Kansas, Colorado, and the Dakotas. The 



soil is composed of dune sand, probably derived from the Loup Fork (Tertiary) 



beds. These hills seem to have been formed largely at some previous epoch 



and to have become stabilized and occupied by vegetation. Through the 



influence of man, mostly on account of prairie fires and overgrazing, many of 



these ancient dunes have become rejuvenated to the detriment of those respon- 

 sible for it. 



After giving the results of his careful measurements of wind, rainfall, 



■ 



evaporation, temperature, and other ecological factors, Pool takes up in 

 detail the vegetation of the region. It is a pleasure to note the author's 

 caution in using the word "formation." He rightly believes in using this 

 term only for large units, referring the "formations" of many authors to asso- 

 ciations. The characteristic upland formation is the prairie-grass formation, 

 which is contrasted sharply w r ith the short-grass formation of the plains, the 

 two embracing most of the great climatic grasslands between our eastern 

 forests and the mountains. These two great formations have similar physiog- 

 nomy but different component species; the limiting factors are the available 

 water and competition, and not temperature, as supposed by Merriam. The 

 chief association is the bunch-grass association, dominated especially by 

 Andropogon scoparius; this is the vegetation that prevailed generally before 

 the advent of the white man, and is regarded as the temporary climax of the 

 region. The vegetation of this association is open, the grasses occurring in 

 tufts or bunches, but it is supposed that ultimately some closed prairie- 

 grass association will prevail. There is evidence of this in the spear-grass 

 association (dominated by Stipa comata and Koeleria cristata), and farther 

 west in the grama buffalo-grass association (dominated by Boiiteloua and 

 Bulb His). 



Doubtless the most interesting features of the sandhills are the blow-outs. 

 These are retrogressive features and are due, as noted above, especially to 

 prairie fires and overgrazing. At first, through the death of the plants there 

 are small patches of bare sand. Later the sand is scooped out by the wind, 

 forming conical or crateriform depressions, known as blow-outs. As the sand 

 is scooped out, more sand falls in from the sides, so that the blow-out is increased 

 in circumference, as well as in depth. Extreme cases are recorded where the 

 depth may be as much as 100 feet and the circumference 600 feet. When 

 wind erosion becomes checked, vegetation again gets a foothold, the chief 



6 Pool, Raymond, J., Glimpses of the Great American Desert. Pop. Sci. 

 Monthly 80:209-235. figs. 17. 1912. 



