192 THE LURE OF THE LAND 



case we have a very great variety of soils belonging to 

 this class. 



" The third class includes all of the alluvial soils 

 formed by deposition from rivers and streams, of which 

 we have such abundant examples about the mouth of 

 the Mississippi. They constitute all the bottom lands 

 of the West, and indeed of the whole country. They 

 also are found in places, particularly in the West, 

 occupying the beds of ancient lakes, a notable example 

 of which is found in the fertile soils of Dakota, popu- 

 larly known as the Red River region. Here was an an- 

 cient lake of very great size, known to geologists as Lake 

 Agassiz, extending southward of Lake Traverse, on Red 

 River, widening northward and extending on both sides 

 of the river, perhaps fifty-five or sixty miles wide ; there 

 its bed leaves the country, expanding to much greater 

 width northward in Manitoba. This tract is exceed- 

 ingly level, the soil of varying depth, very fine, black 

 with the decomposition of vegetable matter, and very 

 fertile. As we proceed westward soils belonging to this 

 class contain less and less vegetable matter, although 

 not necessarily less fertile, until in the valleys of Cali- 

 fornia we have in places soils of great fertility which 

 contain very little vegetable matter (humus). The 

 amount of vegetable matter coloring the soil black 

 depends very largely on the temperature, climate and 

 on the amount of water. There is little of such matter 

 in a dry region or in a region subject to periodical 

 droughts, and yet such a soil may be very fertile in the 

 mineral constituents necessary for grain, and in sea- 

 sons, with sufficient rain or by irrigation, very large 

 crops may be grown. 



" These three classes of soils run into each other by 

 insensible gradations. The classification is given as 



