126 PHYSICAL GEOGKAPHY. 



bottom of the vessel as a layer of sand, while the feldspar, 

 hornblende, and mica also, if present, are suspended for a 

 time in the water and are then deposited as a layer of 

 common clay npon the sand.* Mixing the whole together 

 again we have a handful of common soil in a condition fit 

 for the growth of vegetation. 



To constitute a proper fertile soil the mineral ingredients 

 must of course have an addition of humus, derived from 

 decayed vegetation. This has been abundantly supplied 

 to Iowa soil by the growth of prairie grasses and other 

 vegetation which has nourished upon it during the unnum- 

 bered years that have passed before civilized man disturbed 

 its virgin repose. This accumulated fertility of the past now 

 constitutes our great agricultural wealth ; for it is a fact not to 

 be disguised that our present agricultural prosperity is due 

 to the primitive fertility of our soil, and although it is now of 

 almost marvelous fertility, yet the time is soon to come when 

 we must enquire with an earnestness, excited by necessity, 

 into our resources for its fertilization and restoration. 



The materials of the drift are so nearly uniform from 

 top to bottom, and so 1 great a proportion of these are finely 

 comminuted, that almost any part of it, after proper exposure 

 to the atmosphere and frosts, may be brought to the condi- 

 tion of good soil; so that the depth of any given soil, as 

 the term is generally understood, is indefinite, but it may be 

 said to have the depth to which the fibrous roots of the 

 primitive vegetation has reached, and thus added humus 

 to the mineral constituents. This depth is usually from two 

 to four feet, but no Iowa farmer has any fear of plowing 

 so deep as to reach through his soil. It will thus be seen 

 that the subsoil does not differ materially from the soil 

 proper. Hence no possible objection can be urged against 

 subsoil plowing, while the nature of the subsoil itself suggests 

 its great value. 



*This also well illustrates how stratified rocks are formed and how they derive their 

 materials from the crystaline or unstratified rocks. 



