108 A PRACTICAL TREATISE 



it has proved an excellent manure for clayey soils, mellowing 

 their stiffness, and rendering them easier to work. 



3. Sldhj or slony marl, to which class, also, properly be- 

 longs that which is called rotten limestone, is chietiy applied 

 to iieavy land. Its operation is slow, but very lasting; land, 

 forty years after it has been laid on, having been found to bear 

 a closer and a better crop of grass than that which had been 

 recently applied. 



4. Shelly marl, which is evidently produced by the remains 

 of testaceous fish, which, dying in their shells, become, in 

 process of time, converted into calcareous earth, and their 

 bodies, when decomposed, furnish a kind of mould composed 

 of animal substance, which is no doubt analogous to the effect 

 of dung. It is, therefore, highly fertilizing when judiciously 

 applied to soils of every kind, which are either in themselves 

 dry, or which have been properly drained. 



Such are the most common denominations by which marl is 

 usually distinguished, though it is susceptible of many sub- 

 divisions by those who affect to treat the subject scientifically. 



It is, however, more frequently classed under the sole cha- 

 racters of siliceous, argillaceous or calcareous, according as 

 sand, clay, or lime predominates in its composition ; but for all 

 practical purposes, it may be sufficient to divide it into earth- 

 marl and sliell-marl. 



Earth-Marl. — The former, though in substance, as we have 

 already seen, sometimes principally formed of sand, is yet, in most 

 cases, chiefly composed of clay, and of the carbonate of lime, 

 intimately combined, but mixed in very different proportions, 

 by which its properties are necessarily varied. It acts as 

 manure physically, or substantially, through the effect of the 

 clay in rendering soils tenacious; and chemically, by the 

 operation of lime in the manner which has been explained in 

 treating of that fossil. 



Although it is very generally thought that extreme accuracy 

 in philosophical experiments is useless in the practice of agri- 

 culture, yet it is particularly necessary to ascertain the precise 

 difference between these modes of action; for, of course, either 

 one or the other prevails, according to the greater or the less 

 quantity of clay of which tjie marl is composed. Thus, to 

 produce the first-named, or physical eftect, a much larger 

 amount must be laid upon the land than when the second is 

 the object; for clay can only be advantageously employed in 

 that view upon soils that are too light, and consequently tho 



