THE ORIGIN OF SOILS 31 



The stratified rocks are so called because they are 

 deposited in layers or strata. They are derived from 

 the remains of previous rocks, though not necessarily 

 directly from the unstratified, and frequently are com- 

 posed of non-crystalline minerals. They contain fossils, 

 and are classified by geologists according to age 

 (p. 42). In this country stratified rocks are the most 

 abundant, the soils derived from them are widely distri- 

 buted and reflect their characteristic properties. Sand 

 and clay respectively may be taken as the two chief types 

 of the sedimentary deposits. 



Sand. Sands are found plentifully by the seashore, 

 sometimes in level stretches covered with stunted verdure, 

 sometimes piled up in mounds or dunes fifty or sixty feet 

 high. Inland beds of sand, such as the Bagshot sands, 

 Thanet sands, etc., are also common and cover large tracts 

 on the geological map. It is often said that sand consists 

 of silica, and sometimes that silica is pure sand. Unquali- 

 fied, the statements are not true, and their repetition has 

 led to much confusion. The term sand is commonly applied 

 to quantities of small gritty angular fragments of any 

 crystalline mineral or mixture of minerals. Quartz 

 (crystallised silica) is usually present in larger or smaller 

 quantity, and not infrequently comprises the bulk of the 

 material. Sea sands often consist entirely of quartz - 

 a fact which probably gave rise to the misstatement referred 

 to but inland deposits are generally more mixed in 

 character. 



The size of the particles varies. Sand is a popular 

 rather than a scientific term, and cannot be defined as 

 consisting of grains of any particular size. The only limit- 

 ing property is that vaguely known as grittiness. This 

 property would not be generally recognised in material 

 consisting of particles of less than 0'025 millimetres (say 

 f an inch) or greater than 2*5 millimetres (say -^ of 



