iS THE ORIGIN OF SOILS [chap. 



and Beds., and in some of the Wealden sandstones, or 

 carbonate of lime, as in the Kentish Rag, or even silica 

 itself, as in the hard blocks of tertiary sandstone, which 

 are left as " grey wethers " on the surface of the chalk. 

 In some of the older sandstones the rock is practically 

 homogeneous ; heat, pressure, and solution having 

 thoroughly felted the grains together. Many sand- 

 stones weather rapidly, through the solution of the 

 cement binding the grains together ; the resulting sand 

 has the same texture as it possessed before it was 

 cemented into a rock. 



The grains of sand that are first weathered from a 

 crystalline rock possess an angular shape, but are 

 soon rubbed down in running water into rounded 

 grains with a surface like fine ground glass. Hence 

 the degree of angularity which the sand grains show gives 

 some indication of the amount of wear and tear they 

 have suffered since their origin as sand. Below a certain 

 size, however, quartz grains seem no longer capable of 

 rubbing against one another, but remain angular even 

 after long travel in running water. Daubree has shown 

 that angular fragments of sand of less than cm mm. 

 in diameter will travel in water without becoming 

 rounded, hence any rounding of smaller grains of sand 

 must have been due to solution. 



Silica in the crystalline state is very slightly soluble 

 in water, a certain amount of solution taking place 

 even at ordinary temperatures : most natural waters 

 show a little silica in solution, though this more 

 probably arises from the decomposition of natural 

 silicates by water containing carbonic acid, rather than 

 from the direct solution of quartz. 



Amorphous silica in the form of "flint" plays a 

 conspicuous part in the constitution of many soils in 

 the south and east of England ; owing to their 



