3 8 THE MECHANICAL ANALYSIS OF SOILS [chap. 



identified. In fine-grained sands the fragments of 

 minerals other than quartz become as a rule more 

 abundant, till they begin to predominate over the 

 quartz grains in the finest silts and muds that are 

 deposited from very gently moving water. Under the 

 microscope the quartz grains show a crystalline struc- 

 ture, and a surface more or less dulled and rounded 

 according to the travel the grains have suffered. In 

 mass the chief characteristic of sand is its want of 

 coherence when dry. 



Clay. The material we call clay is characterised by 

 certain properties that are shown when the clay has 

 been "puddled," i.e., kneaded when in a moist condition. 

 The clay is plastic, it can be moulded and worked into 

 various shapes, even into quite thin leaves, and it will 

 retain these shapes on drying. During the drying 

 process a shrinkage takes place: the dry material is 

 hard and tenacious, and can only be broken or crumbled 

 with difficulty. The shrinkage is considerable : a little 

 brick was made of good modelling clay 7 inches long, 

 and about 1 square inch in section ; two marks were 

 then made on this 6 inches apart ; after a fortnight's 

 drying in a room the marks were only 5-7 inches apart, 

 showing a shrinkage of 5 per cent. Clay is further 

 impermeable to water when in the moist puddled 

 condition, for which reason it is used to line the bottoms 

 of ponds in pervious soil, and is built up inside the 

 retaining dams of reservoirs ; quite a thin layer of clay 

 will hold water indefinitely as long as it is not allowed 

 to dry and crack, nor to be washed away by the action 

 of running water. 



From a chemical point of view, all clays are found 

 to consist largely of kaolinite, the hydrated silicate 

 of alumina which is formed by the weathering of fel- 

 spar ; the other materials present consist of extremely 



