252 MATERIALS OF INDIRECT VALUE [cHAP. 



Since lime becomes calcium carbonate in the soil, 

 obviously the same results would be obtained by 

 applying the latter material, the main advantage in the 

 use of lime lies in the very fine state of division into 

 which it falls on slaking and the consequent good 

 admixture with the soil that is effected. 



Such a finely divided calcium carbonate is provided 

 in many parts of the country by the calcareous marls 

 which occur in beds sufficiently near the surface to 

 admit of working, as in the New Red Sandstone 

 formations in Cheshire, Worcester, etc., or the shell 

 marls which occur in Norfolk, A true marl is a clay 

 containing a variable percentage of calcium carbonate> 

 it is specially valuable on sandy or peaty soils, not only 

 for its calcareous matter but also for the clay, which 

 improves the texture of the soil. 



In many parts of the country, where the superficial 

 formations resting upon the Chalk are \ devoid of 

 carbonate of lime, it was formerly the custom to sink 

 bell pits into the chalk rock, haul it up in baskets and 

 spread it upon the surface. In Hertfordshire, for 

 example, this chalking was part of the regular routine 

 of farming from the earliest times of which we have 

 records, and from the analyses of the Rothamsted soils 

 it has been ascertained that by the repetition of the 

 process a hundred tons per acre or more must have 

 been applied before the beginning of the nineteenth 

 century, there being now present in the soil from 

 2 to 5 per cent, of carbonate of lime, all of artificial 

 origin. Chalk was also formerly carried for con- 

 siderable distances on to the clay formations — the 

 London Clay, the Gault Clay, and the Weald Clay — 

 that are contiguous, but the increased cost of labour 

 has put an end to this practice. 



None of the other British limestones are sufficiently 



