l] transport of WEATHERED MATERIAL 15 



surface. Darwin found in one case that a layer of burnt 

 marl spread on the surface had sunk 3 inches in fifteen 

 years, in another case a layer of chalk was buried 7 

 inches after an interval of twenty-nine years ; in neither 

 case, however, can we estimate the part played by the 

 accretion of dust in forming this deposit. When we 

 consider for how long a period worms must have been 

 working in our cultivated soils, it is clear that the whole 

 must have been through them over and over again, and 

 that much of the fineness of the surface soil must be due to 

 their action, both in actually grinding the fragments and 

 in constantly bringing the finest portions back to the top. 



In addition to the alluvial deposits proper, which are 

 still in process of formation, beds of gravel, sand, and 

 brick earth occur in many river valleys, as terraces on 

 the flanks of the hills, often much cut and denuded by 

 the modern river. These high level formations prob- 

 ably represent alluvial deposits of a former epoch 

 where the general slope of the land was greater and the 

 rivers, fed by a higher rainfall in the hills, ran in greater 

 volume. That the material of which these deposits con- 

 sist has been sorted by running water is evident from 

 the uniformity of size it possesses in each bed : while 

 the coarseness of the gravel, and the fact that in some 

 cases the stones are not made from the immediately 

 underlying rock, all point to a great lapse of time and a 

 river of higher transporting power than the present one. 

 The wide deposits of brick earth in the neighbourhood 

 of London and in East Kent were probably laid down 

 either by floods on the river meadows or in quiet bays 

 and lagoons of an estuary. 



Over a great part of Britain north of the Thames, 

 especially in the midlands and the eastern counties, 

 the surface of the land is covered with beds of clay 

 and sand which owe their origin to glacial ice. In 



