46 PLANT GEOGRAPHY 



i.e. the percolation of rain-water, frost, and fluctuating 

 surface-heating; but the roots of plants and burrowing 

 animals, especially earthworms, take an important part 

 in the process so far as the soil is concerned. Soils may, 

 in this way, originate in situ, from the disintegration of 

 the underlying rock, when they are termed local or 

 sedentary ; or they may have been transported from their 

 place of origin, as is the case with loess and the blown 

 sands of dunes (eluvium], the boulder-clay and gravel of 

 the Glacial Period (diluvial), and the brick-earths and 

 gravels of lake and river-basins (alluvium). 



THE PHYSICS OF THE SOIL. The physical properties 

 of soil, as distinguished from those that are chemical, 

 are chiefly its depth, its texture, including the size of its' 

 particles, its " pore-space," density, tenacity, water 

 capacity, capillarity, 'permeability, and temperature. If 

 the soil be shallow the roots of trees may penetrate it, 

 and their nutrition receive a check on their contact with 

 the more compact and less nutritious subsoil. Shallow 

 soil is also liable to drought, and is, therefore, charac- 

 terised by xerophytic vegetation. Owing to their 

 greater resistance to weathering, the older, or Palaeozoic, 

 formations commonly yield shallower soils than those 

 of more recent geological age. 



The size of the particles in a soil may vary very much, 

 the larger ones being known as "stones"; but the 

 density, pore-space, and tenacity of a soil do not 

 depend only upon the fineness of its particles. If these 

 are of uniform size it makes no difference to the volume 

 of the spaces between them whether they be large or 

 small. Particles of the same size, however, may be 

 loosely or compactly arranged so as to have much or 

 little pore-space between them; whilst if smaller par- 

 ticles be intermixed with larger ones, the pore-space may 

 be indefinitely reduced. It has been calculated that 

 with grains one millimetre in diameter there would be 

 approximately 700 grains in a gram of ordinary soil, 

 this number varying inversely as the third power of the 

 diameter of the grains. Thus with a diameter of o.i 

 mm. there would be 700,000 grains to the gram. The 

 total area of the surfaces of these particles varies in- 

 versely as their diameter, a sphere one inch in diameter 

 having only half the surface of eight spheres of half- 

 inch diameter, though the volumes would be equal. This 



