


578 STRUCTURAL AND FIELD GEOLOGY 
reason, coarser grained materials become concentrated in the 
soil, which thus tends to acquire a coarser and more open texture 
than the subsoil, more especially under moist climatic condi- 
tions. The rate at which a soil wastes away varies indefinitely. 
Where the ground is flat and thickly clothed with vegetation, 
there may be little waste, while, owing to the action of worms 
continually bringing up fine-grained materials to the surface, 
the soil may come to show a finer texture than the subsoil. 
Other things being equal, however, surface-waste naturally 
increases with the slope of the ground, and is greater when 
the soil is bare than when it is well carpeted with verdure. 
As absolutely flat ground can hardly be said to exist, surface- 
waste is everywhere in progress, on steep and gentle slopes 
alike. Slowly or more rapidly, as the case may be, dis- 
integrated rock-material is continually being urged forward, 
and eventually finds its way into brooks and rivers. In this 
way the whole surface of the land is gradually lowered. 
How effective such action has been, may be illustrated by the 
occurrence upon plateaus and flat hill-tops, of rock-fragments 
derived from thick formations which formerly overspread 
those regions, but have now entirely disappeared. As an 
example, we may cite the “grey-wethers” or “sarcen-stones ” 
that often occur in the soils of the Chalk Downs. These 
fragments of siliceous sandstone are the relics of certain 
Tertiary deposits, which at one time covered wide areas in 
southern and south-eastern England. During the slow 
srowth and waste of the soil-cap the Tertiary deposits 
referred to have been gradually but persistently removed, 
the “sarcen-stones” (owing to their size and their insoluble 
character) being the only recognisable relics left behind. In 
this way, notwithstanding the persistence of a _ soil-cap 
through long geological ages, the whole surface of a country 
has nevertheless been lowered for many hundreds of feet. 
The great variety of bed-rock soils may be illustrated by 
a short description of those met with on certain well-known 
types of rock. 
Soils from Igneous Rocks.—The soils derived from the 
disintegration of igneous rocks necessarily vary in character. 
It will suffice, however, to cite a few typical examples. 
Granite-—The weathering of this rock has already been 

