320 ECOLOGY 



illustration is the society of Soft Grass, Bracken, and Blue- 

 bell so common in the Sessile Oak wood associations. 



The more important plant-associations indicated should, 

 wherever possible, be studied in the field ; but much may 

 be learnt from a detailed study of those types of vegetation 

 which lie close at hand, e.g. the plants of a hedge and ditch, 

 a meadow, a wooded escarpment, or a bit of moorland. 

 As a preliminary to the study of plant-associations we will 

 make a few observations on soils. 



CHAPTER XXVI 



THE SOIL 



Origin of soils. Sedentary and transported soils. If we 

 examine a section of soil in a quarry, as in Fig. 212, we can 

 form some idea of its origin. At the surface is a dark layer 

 containing the roots of the plants forming the surface vege- 

 tation. Below this is a lighter layer, the subsoil, which 

 grades off into the hard rock beneath. Acted upon by the 

 atmosphere, rain, and frost, the upper parts of the rock have 

 been broken into fragments, the smallest particles being 

 nearest the surface and forming the soil. Such a soil has 

 been derived from the rocks below, and its surface layer has 

 been darkened by organic matter, chiefly the decaying 

 remains of plants, which have grown in it. A soil of this 

 kind is said to be sedentary. 



A section along a river bank is very different. Often down 

 to a considerable depth we find no trace of rock from which 

 the soil could have been formed. The soil is made up of 

 particles varying in size from fine grains of sand to pebbles 

 and even boulders, all more or less rounded and water- 



