THE LIFE HISTOKIES OF LAKES. 95 



more lasting tlian those left by lakes in humid lands, for the reason that 

 the climatic conditions are less destructive. The terraces and embank- 

 ments of gravel left by lakes in desert valleys are especially permanent 

 t()i)Ogra[)hic features, as the scanty rain that falls on them is absorbed and 

 allowed to percolate slowh' through them, instead of flowing down their 

 surfaces so as to erode. The sediments deposited in enclosed basins are 

 also protected from destruction, as they cannot be removed by streams 

 until some change inaugurates free drainage to the sea or to some lower 

 Ijasin. A continuation of aridity' in a desiccated lake basin, results 

 normall}' in the burial of the lacustral sediments beneath subaerial 

 deposits, thus again insuring their preservation. To folloAV this subject 

 farther would lead to a comparative study of the processes of erosion in 

 arid and in humid regions, which is beyond the scope of the present essay. 



It will be seen from what has been presented above wdth reference to 

 the normal course of the lives of lakes, that in spite of the many varia- 

 tions they present, the seeds of death are planted at their birth, and they 

 are destined, sooner or later, to pass away and give place to other condi- 

 tions. 



Interruptions of the even tenor of the lives of lakes, in both arid and 

 humid regions, such as the effects of upheaval and depression of the earth's 

 crust, earthquakes and volcanic eruptions, might be considered, but 

 these abnormal incidents, like the accidents in human lives, cannot be 

 foretold, and apply to individuals rather than to classes. 



