374 CLAY MARL AND SHELL MARL. 



cent. When much richer, say near or quite 80 per cent., it becomes 

 of stony hardness, or passes into lime-stone. 



In Britain the marls most abounding in clay are called "clay 

 marl," and vulgarly " clay" simply. This is the kind most gene- 

 rally used, and in enormous quantities. Stephens (in the latest 

 edition of his "Book of the Farm"), offers the first precise in- 

 formation that I have seen, as it is also the most recent of the 

 component parts of this marl, as follows : from Johnston on the 

 Use of Lime. 



4. " The following analysis may give a fair idea of the composition of a 

 clay marl." This specimen was found in Ayrshire. 



Carbonate of lime . 8.4 



Oxide of iron and alumina 



Organic matter 



Clay, and silicious matter 



Water 



2.2 



2.8 



84.9 



1.4 



99.7 



Every one who has observed what is called marl in lower Vir- 

 ginia will recognise its entire disagreement with the true marl 

 described in all the foregoing quotations, in every physical or 

 mechanical property, in texture, and in its manifest origin or for- 

 mation. 



5. " Shell Marl. In some parts of the country, as in Forfarshire [Scot- 

 land,] this substance is found in great quantities associated with peat. 

 . . . . It is taken out of the bogs by means of a boat mounted with a 

 dredging apparatus. When of fine quality and in a dry state, it is as 

 white as lime, not crumbling down into powder like quick-lime, but cutting 



something like cheese, with the spade It is applied at 40 to 



50 bolls (8 cubic feet) to the acre. When applied as lime, it is beneficial; 

 but, as is often the case, when applied solely as manure, in quantities of 35 

 to 45 cubic yards to the acre, it never fails to be mischievous. It does not 

 easily injure new fresh land ; [Qu. the first time applied ?] but when re- 

 peated frequently, as a sole manuring, I have seen land reduced to such a 

 state of pulverization, that the foot, with a stamp, sank into the ground as 

 deep as the ankle. Applied to lands followed by severe cropping, it has 

 reduced them to a state of utter sterility, which they have not recovered 

 from to this day." (Stephens' Book of the Farm, or Farmers' Guide, 1850; 

 Headrick's Survey of Forfarehire.) 



This " shell marl" consists of Top of bed. Bottom of bed. 



" Carbonate of lime . . . . 77. G 81.7 



Oxide of iron and alumina ... 1.8 0.6 



Organic matter . . . . . 14.6 14.6 



Insoluble, chiefly silicious matters . 6.0 3.1 



100. 100. 



This substance, according to its analysis above, is undoubtedly 

 the most valuable of all calcareous manures. But still it is nob 



