146 HOW CROPS FEED. 



heavy masses, alluvium is always more or less sti'atijied 

 or arranged in distinct layers: stones or gravel at the 

 bottom and nearest the soiirce of movement, finer stones 

 or finer gravel above and furtlier down in the path of 

 flow, sand and impalpable matters at the surface and at 

 the point where the stream, before turbid from suspended 

 rock-dust, finally clears itself by a broad level course and 

 slow progress. 



Alluvial deposits have been formed in all periods of the 

 earth's history. Water trickling gently down a granite 

 slope carries forward the kaolinite arising from decompo- 

 sition of feldspar, and the first hollow gradually fills up 

 with a bed of clay. In valleys are thus deposited the 

 gravel, sand, and rock-dust detached from the slopes of 

 neighboring mountains. Lakes and gulfs become filled 

 with silt brought into them by streams. Alluvium is 

 found below as well as above the drift, and recent alluvium 

 in the drift region is very often composed of drift mate- 

 rials rearranged by water-currents. Alluvium often con- 

 tains rounded fragments or disks of soft rocks, as lime- 

 stones and slates, which are more rarely found in drift. 



ColIUTial Soils, lastly, are those which, while consisting 

 in part of drift or alluvium, also contain sharp, angular 

 fragments of the rock from wliich they mainly originated, 

 thus demonstrating that they have not b('en transported 

 to any great distance, or are made u]) of soils in place, 

 more or less mingled with drift or alluvium. 



§2. 



DISTINCTIONS OF SOILS BASED UPON OBVIOUS OR EXTER- 

 NAL CHARACTERS. 



The classification and nomenclature of soils customarily 

 employed by agriculturists have chiefly arisen from con- 

 sideration of the relative proportions of the principal 



