178 AMERICAN MANURES. 



of lime, supposed to be derived principally from 

 the shells of fishes. The amount of carbonate 

 of lime is usually from 10 to 20 per cent. There 

 is a great variety of substances designated as 

 marl, but they may all be reduced to four general 

 kinds : clay, stone, shell, and green-sand marl. 

 The first very nearly resembles clay ; this variety 

 usually contains from 10 to 20 per cent, of car- 

 bonate of lime, and its value as a manure must 

 be attributed to the action of that substance. 

 Stone marls are generally richer in carbonate of 

 lime than the clay varieties, and are a nearer 

 approach to the ordinary carbonate of lime. 

 Shell marl contains a large amount of shells 

 partially decomposed ; its action on soils is more 

 immediate, because it is more soluble than the 

 clay and stone varieties. Green-sand marl is 

 most valuable of all; it generally contains a 

 small percentage of carbonate of lime, seldom ex- 

 ceeding 5 per cent., while it contains potash in an 

 insoluble condition in amounts varying from 2 

 to 7 per cent. It is by some considered quite as 

 valuable, weight for weight, as wood ashes; it 

 also generally contains a small amount of in- 

 soluble phosphoric acid, and its application on 

 what were considered worn out lands has been 

 followed by remarkable results. Where these 

 marls abound, they are a cheap and efficient 

 manure for the farmer, and should not be 



