ALLUVIA. 621 



with deposits of this substance, often of great 

 depth. 



In consequence of the daily waste of moun- 

 tains, and without the assistance of streams, but 

 urged solely by the force of gravity and the ordi- 

 nary drainage of the surface, beds of alluvial 

 matter are in many situations deposited on the 

 sides of hills, and in some cases to a considerable 

 depth. 



Lastly, in some places, rocks are found to 

 have been disintegrated in situ in consequence of 

 the action of water or other causes, and sometimes 

 to a considerable depth. In favourable circum- 

 stances of position, these remain in their places, 

 forming beds or masses of loose materials, con- 

 sisting of larger fragments of the more solid 

 parts, mixed with the clay and sand resulting 

 from the more complete decomposition of others. 

 This occurrence takes place, chiefly, in granite, 

 in gneiss, in some of the trap rocks, and in the 

 red sandstone. Of the latter, Arran and Cantyre 

 present some remarkable examples. 



Thus there seem to occur in nature four dis- 

 tinct classes of partial alluvia, namely, those of 



