THE SulL 87 



The soil left on limestone or chalk consists mainly of these 

 foreign substances, most of the calcium carbonate itself having 

 been dissolved out by the combined action of water and carbon 

 dioxide. It sometimes happens that the soil on limestones 

 would be benefited by the application of manures containing 

 lime. 



Limestone only exerts its characteristic and important 

 functions in a soil when in a very finely divided state; in the 

 form of gravel or sand it is little better than ordinary siliceous 

 sand. In the finely divided condition it has two very valuable 

 functions — as a source of plant food, by virtue of the phosphates, 

 sulphates, and calcium which it contains, and, what is more 

 important, as a basic material necessary for the processes of 

 nitrification. 



Sedentary and Transported Soils. — These terms are con- 

 venient in distinguishing between soils which are made up of 

 the debris resulting from the weathering of the particular 

 rock on which they rest (sedentary soils), and those which owe 

 their origin, fiot to the rock below them, but to matters brought 

 from a distance and deposited there (transported soils). The 

 rich alluvial soil in the lower reaches of river valleys consists 

 largely of matter which has been brought down by the river 

 from the higher parts of the valley, and since the materials have 

 in many cases, been brought from various rock formations the 

 resulting soil generally possesses a greater fertility than would 

 be shown by a soil formed exclusively by the weathering of 

 any one kind of rock. 



Other excellent examples of transported soils are afforded by 

 the " warp " soils of the Humber and Trent. In these cases 

 the soil itself is transported, and not merely the materials out 

 of which it is (afterwards) formed. 



Glaciers are also the means of transporting large quantities 

 of materials out of which soils may be formed. Large tracts of 

 country are covered with a thick deposit of clay and rock frag- 

 ments which have been brought from a great distance bj 



