146 HOW CROPS FEED. 
heavy masses, alluvium is always more or less stratified 
or arranged in distinct layers: stones or gravel at the 
bottom and nearest the source of movement, finer stones 
or finer gravel above and further-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. 
Colluvial Soils, lastly, are those which, while consisting 
in part of drift or alluvium, also contain sharp, angular 
fragments of the rock from which they mainly originated, 
thus demonstrating that they have not been transported 
to any great distance, or are made up 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 
