CHAP. II.] 



THE NA TURE OF SAND 



37 



are of most importance, since soils which are mainly 

 characterised by chalk or humus are less commonly 

 in cultivation. 



Sand. On the seashore, in beds of an alluvial 

 nature, and in formations of all geological ages, we are 

 familiar with sand ; in the main it consists of grains of 

 quartz, rounded by continual rubbing, and more or less 

 :oloured by oxide of iron. It represents the quartz 

 :ontained in the fundamental rocks, weathered and 

 vorn by water : in some cases of comparatively 

 recent origin, in others it is material that has re- 

 peatedly been formed into a sedimentary rock, dis- 

 ntegrated afresh and' sorted by the action of running 

 vater. The coarser the grains of which a sand is made 

 ip, the more rapid must have been the current from 

 vhich it was deposited. The following table shows 

 he rate of flow which is necessary to carry sand 

 rrains of various sizes : 



A closer examination of most sands will show that 

 ley do not consist wholly of quartz grains, but also 

 ontain rounded fragments of many of the minerals 

 resent in the fundamental rocks which have any 

 esistance to weathering. Flakes of mica are common, 

 agments of more or less altered felspar, of oxide of 

 on, and even of tinstone, rutile, and zircon, may be 



