52 



o/'.sj// carried to the Mexican Gulf by the Mississippi, according to the Delta Survey 

 under Humphreys and Abbot, is about T-^TTT of the weight of the water, or ^V,, of its 

 bulk ; equivalent for an average year to 12,500,000,000,0'iO pounds, or to a mass 1 

 square mile in area and 241 feet deep.* 



These constitute the "bottom lands," as they are called in the West. 

 The Ked River region, which has become famous as a wheat producing 

 country, lying partly in Minnesota and partly in Dakota, occupies .the 

 bed of an ancient lake, known to geologists as Lake Agassiz, and is 

 composed of a black sedimentary soil, exceedingly tine in texture, and 

 very fertile and deep. This tract extends southward to Lake Traverse, 

 on the Ked River, widening as it proceeds northward and extending on 

 both sides of the river 50 or 60 miles wide where its bed leaves this 

 country, and expanding to much greater width in Manitoba. 



The further westward soils of this class are found the less the amount 

 of organic matter they contain, although the soils are not necessarily 

 less fertile, until in some places in the valleys of California are found 

 soils of great fertility which contain an exceedingly small amount. Of 

 course such soils, as those of California just mentioned, are deficient in 

 the faculty of storing up water for future use, and, however rich they 

 may be in mineral constituents, yet in a dry region or one subject to 

 periodical droughts, irrigation would have to be resorted to in order to 

 get large yields of crops. 



Soil* of disintegration. These occupy the undulating parts of this 

 country lying south of the drift, possessing every variety of character, 

 both in regard to their chemical composition and physical properties, 

 as their mode of formation indicates, arising from the disintegration 

 of the subjacent rocks by atmospheric agencies. 



Where the underlying rock has been an impure limestone, containing 

 much insoluble matter, the carbonate of lime has been slowty dissolved 

 out by the action of the carbonic acid contained in the rain, leaving the 

 insoluble matter behind. Such soils as that of the "blue grass" regions 

 of Kentucky are so formed, and are often of extreme fertility. (See 

 the "Kentucky Geological Reports" for further details about this re- 

 gion, including the chemical analyses of its soils*) 



Professor Whitney states that some of the prairie soils of Iowa, par- 

 ticularly those where the soil is almost of impalpable fineness, have 

 been produced by the slow action of atmospheric agencies on beds of 

 limestones which formerly occupied their places. In the course of time 

 the soluble carbonate of lime was gradually dissolved out and carried 

 away by the rivers and streams to the ocean, and a small amount of 

 insoluble residue was left, forming the thick prairie soil of the region, 

 which has since become blackened by the decay of subsequent abundant 

 vegetation on it.t 



In the table-lands of Oregon and Washington the underlying rock is 

 volcanic, and the soil arising from its disintegration is very fine in texture, 



* Dana's Geology, p. 648. 



t Iowa Geological Survey, Vol. I, 1858. 



