CHAP. II.] 



THE NATURE OF SAND 



33 



to which the texture of the soil has been referred — the 

 sand, clay, chalk, and humus — of which the first two 

 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 

 coloured by oxide of iron. It represents the quartz 

 contained in the fundamental rocks, weathered and 

 worn by water : in some cases of comparatively 

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

 peatedly been formed into a sedimentary rock, 

 disintegrated afresh and sorted by the action of running 

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

 up, the more rapid must have been the current from 

 which it was deposited. The following table shows 

 the rate of flow which is necessary to carry sand 

 grains of various sizes : — 



A closer examination of most sands will show that 

 they do not consist wholly of quartz grains, but also 

 contain rounded fragments of many of the minerals 

 present in the fundamental rocks which have any 

 resistance to weathering. Flakes of mica are common, 



C 



