PREFACE. 



By D. T. MacDottgal. 



The geological and biological processes of the surface layers of the earth's crust are 

 profoundly different under subaerial exposures to those which prevail under subaqueous 

 conditions. Any region, therefore, which may be subjected to submergence and to weather- 

 ing, alternately, will offer a changing complex of environic conditions, with accompanying 

 disturbances in the balance and distributional movements of the organisms of the region. 



The solid ground, in one case, is subject to the erosion of precipitation-water and 

 stream-flow, coupled with a leaching action of the water, while surface material will be 

 variously transported and deposited. Wind effects will vary widely with aridity and other 

 climatic features, while precipitation, evaporation, and temperature will act as determi- 

 nants as to the character of the living forms supported. 



The submersion of any area may be expected to be followed by the complete or nearly 

 complete destruction of the land flora and fauna. The temperature of the substratum is 

 equalized, variations in moisture disappear, wind comes in only as an agency for transport- 

 ing propagative bodies and as a cause of wave-action, sorting, wearing, and depositing 

 material along shore lines. The deposition of sedimentary material in submerged areas 

 under lakes takes place in such manner as to be easily distinguishable from the effects of 

 stream-action. 



The superposition of the two groups of effects in any region such as the lowermost 

 part of a desert basin, especially when submergence and desiccation alternate at intervals 

 of sufficient length to give full force to the two extremes, might be expected to offer highly 

 unusual physical conditions of the surface layers and of the soil, to which organisms 

 would be expected to display reactions of interest and importance useful in the interpre- 

 tation of phytogeographical phenomena in general. 



The great Cahuilla Basin, which lies to the westward of the lower or southern part 

 of the main delta of the Colorado River, has been the scene of alternations of the kind 

 in question. The lower part of this basin has been submerged and desiccated many times 

 in the last few hundred years, as attested by the numerous beach or strand formations 

 and layers of travertine on the shores. The making of the lake in 1904, 1905, and 1906, 

 at the time of the organization of the work of the Desert Laboratory, offered opportuni- 

 ties for some studies the results of which are presented in the present paper. The facts 

 to be taken into account were so diverse in character, and the necessary methods of cali- 

 bration and estimation so unlike, that the cooperation of a number of workers in various 

 branches of science was enlisted. 



First of all the senior contributor was so fortunate as to secure a general sketch of 

 the geology and topography of the basin by the late Professor William Phipps Blake, 

 whose barometric measurements as a member of the Williamson expedition in 1853 first 

 established the fact that the region was an inclosed basin, the lowermost part of which 

 was below sea-level. The history of earlier travel and the general geography of the basin 

 is described by Mr. G. Sykes, geographer of the Desert Laboratory. The analyses of water 

 samples have been carried out under the direction of Professor R. H. Forbes, at the Agri- 

 cultural Experiment Station of Arizona, by the aid of Dr. W. H. Ross and Professor A. E. 

 Vinson. The surface geology, with especial reference to the soil formations, has been the 

 subject of much painstaking examination by Mr. E. E. Free, formerly of the United States 

 Bureau of Soils. Professor G. J. Peirce, of Stanford University, has contributed the results 

 of studies of organisms living in brackish and saline waters, which are of especial concern 



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