S THE ORIGIN OF SOILS [chap. 



as we descend ; if it be gravel, the stones continue of 

 the same size ; if brick earth, neither stones nor hard 

 stratified clay make their appearance. Should the 

 exposed section be deep enough, we find at last the 

 subsoil suddenly giving place to entirely different 

 material solid chalk, or massive clay, or sandstone, as 

 the case may be perhaps incapable, when disintegrated, 

 of furnishing the stuff of which the upper stratum of 

 gravel or brick earth is composed. In this upper 

 stratum we see the clearest evidence of the action of 

 water ; the brick earth is free from stones and is of even 

 texture, the gravel contains hardly any fine material, 

 and its constituent stones are worn and partly rounded ; 

 only running water can thus sift the heterogeneous 

 results of the weathering of rocks, and grade them into 

 different positions. From what can be seen of the 

 present work of the river, it is clear that the brick earth 

 was deposited where the water was moving very slowly, 

 in quiet bays and in cut-offs, which only from time to 

 time get filled up with muddy flood water ; the gravel 

 must have been laid down in the strongest wash of the 

 currents. 



Soils and subsoils of this type, which bear no 

 particular relation to the underlying rocks, but have 

 travelled from a distance by means of running water 

 or moving ice or wind, are known as soils of transport, 

 or, to use the terminology of the Geological Survey, as 

 drift soils. The photograph, Fig. I, shows the section 

 of a deposit of glacial drift upon magnesian limestone, 

 in which the abrupt change from the stratified rock to 

 the travelled material giving rise_ to the soil is very 

 marked. 



