ON MANURES. " 109 



marl must be laid in proportionate abundance, or it will not 

 improve tiie condition of the ground; whilst a clayey soil 

 would, on the contrary, lose some of its good qualities by the 

 addition of marl, after the eflects of the lime were exhausted. 

 The intimate combination of these two substances in the com- 

 position of marl, atlbrds it, however, this advantage — that it 

 divides, and falls to powder, with greater ease than can be 

 effected by any artificial mixture, and therefore unites more 

 readily with the soil. 



On the other hand, if the calcareous matter in the marl be 

 combined with sand instead of clay, or that there are, as in 

 many instances, veins of calcareous sand intermixed, then it 

 suits a clayey soil. The proportion in which these substances 

 are combined is, however, so different, that they often vary in 

 the* same vein, and it is generally found that the bottom part is 

 more calcareous than the top. From 15 to 40 per cent, is not 

 unfrcquently the proportion of Calcareous matter found in clay; 

 that of a sandy nature generally contains a larger proportion.* 



The stone marl of hilly countries is frequently still more 

 abundant in calcareous substance; but it also, in many other 

 places, contains such large quantities of extraneous matter, 

 that it may be properly considered as belonging to the earthy 

 species, and has, in some instances, been laid upon the land to 

 the extent of 400 to 600 single horse cart-loads per acre, 

 which heavy labour renders the use of lime more economical, 

 although carrried from a greater distance, except in cases 

 where the chief object is to loosen very stiff clays, on which it 

 acts with considerable effect. 



SheU-7narL — Shell-marl is usually of a bluish colour, soft to 

 the touch, and somewhat resembling potters' earth ; but when 

 exposed to the air, it crumbles and tails into a pow^der, nearly 

 in the same manner as lime does in slaking. 



The nature of this marl is very different from that of earth 



* Argillaceous marl usually contains from 68 to 80 per cent, of clay, and 

 from 32 to 20 per cent, of calcareous matter : but it has been found composed 

 of 70 per cent, of calcareous, and S to 10 of sand, with clear signs of some iron. 

 Siliceous marl very often contains above 75 per cent, of sand, consequently 

 chalk and sand are the predominant ingredients. — Kirwan on Manures, 

 p. 13. 



The analysis made by Von Thaer, of a quantity dug out out of pits at Ol- 

 d^iibur;;, in Germany, showed it to contain in 100 parts — 



Of tine sand 36 



Clay of a soapy kind 44 



Mould • . 5 



Carbonate of lime 14 



Gypsum 1 



