OVER-IRRIGATION AND ALKALI 



389 



days, salt lakes, similar to the Great Salt Lake, were no 

 doubt formed, which were dried by the changing climate, 

 leaving great masses of salt, that were later covered by 

 washings from the hills. In other cases, silt and other 



Fia. 114. Quaternary Lakes of the Great Basin. Sources of alkali deposits. 



soils of early geological days were deposited in the presence 

 of salt or brackish water. These in time became hardened 

 into rocks. Consequently, over much of western America 

 there are great exposures of shales and sandstones con- 

 taining large percentages of water-soluble substances. 



