PROBLEMS OF THE DESERT. 3 I 



The mere checker-board arrangement of habitats, now 

 or ten years hence, after the usual descriptive methods would 

 reveal nothing of unusual importance in these areas. An an- 

 alysis of the conditions, of quite another character has been 

 projected with a fair hope of making an adequate interpreta- 

 tion of some of the major distributional movements. This 

 will necessitate the closest observation in following the re- 

 cession of shore lines five to a dozen miles down the slopes 

 of the basin, during the next decade, making numerous 

 measurements and estimations of the nature and rapidity of 

 the movements and responses of the plants. The recession 

 may not be expected to take place at a constant rate, as a 

 result of which important changs in the mechanical composi- 

 tion of the emersed soil will be made. The waves will exca- 

 vate terraces, forming dunes and barriers, sorting ma- 

 terial which it leaves behind In separate layers, each being 

 suitable for different formations of plants. As the result of 

 such previous action, the ancient beach lines thousands of 

 years old are still marked by the bands of vegetation which 

 follow them. So much for the purely physical disturbances. 

 Still another condition Is to be taken into account. The 

 basin Into which the water was poured is a patchwork bowl, 

 with Irregular portions of its wall variously charged with 

 salts as a result of long-continued leaching action. The water 

 has, however, uniformly saturated the surface layer of soil 

 of the submerged area with a solution containing about one- 

 third of one per cent of salts. It Is to be seen, therefore, 

 that the plants growing on the unsubmerged portion of an 

 area containing one per cent of alkali will not immediately 

 follow down the beach in the reoccupation of the ground left 

 bare by the receding waters. Before this is done slow weath- 

 ering and incessant capillary action rriust operate to the re- 

 dlfterentlatlon of the areas as to soil salts, and many occu- 

 pations and re-occupations of the surface by other forms must 

 intervene. 



