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THE FARMER'S MAGAZINE. 



stances on the face of the ground arise from the finer 

 state in which the materials exist, and that much of the 

 lienefit derived is owing to the attention that is bestowed 

 on this point. 



Nature has produced bodies in different forms and 

 qualities, minute in bulk, diminutive, reduced in size, 

 concreted and aggregate, and hard or soft, fusible or 

 nitreous, harsh or sweet, acid, acrid, or corrosive. 

 Some are more simple than others, or they are formed 

 of fewer elementary substances, and are consequently 

 capable of exercising a chemical action on all substances 

 in contact, for all bodies of simple constituents have 

 an aptitude to enter into combinations and to effect de- 

 compositions, and the results will depend on the strength 

 of the respective actions. In making a change in the 

 nature of bodies, fire and water are the two effective 

 agents : the former banishes the existing qualities, and 

 confers new properties ; the latter by its insinuating 

 power disintegrates the mass, and sunders the particles, 

 and brings the elements into a fresh contact in an en- 

 tirely different form. For the purpose of being used as 

 manuies, most bodies require to be altered in some way, 

 by means of which the quality is brought nearer to the 

 condition which is known to yield the desired purpose. 

 Hardened or rocky substances are acted upon by five, 

 and earthy matters are impregnated with liquids. Other 

 similar ways are used to prepare the various matters 

 that are known to form the useful substances. The ma- 

 terials of top dressing are in three divisions : 



Section 1. Earthy bodies. 



2. Animal matters. 



3. Saline substances. 



1. Marl occupies a prominent place among natural 

 substances that do not require a preparation to be used 

 as a top dressing for pasture lands. The word means 

 " marrow, fat, or a preparation of the earth," and by 

 the ancients it was called " terrre adeps," and was well 

 known to them. It is a calcareous earth, or a carbonate 

 of lime with portions of other earths, and assumes a fat 

 unctuous appearance after crumbling or dissolution. 

 Geology places it in the tertiary formation, resting upon 

 the gypsum and alternating with it ; the bed is white 

 and calcareous, and contains silicified remains of trees, 

 plants, and shells : other beds are argillaceous, and of 

 great thickness, and often contain balls of celestine or 

 sulphate of strontites. The uppermost beds are thin, 

 and contain large quantities of oysters ; marine shells 

 abound in the beds that join with sands and sandstones, 

 and the last bed is of the fresh-water denomination. 

 Marls have been found stratified in some parts of 

 Europe. 



The component, parts of marl are so very minutely 

 blended as to be invisible to the naked eye ; and from 

 this circumstance, and from containing both salt and 

 fresh-water organic remains, and from the fissile 

 structure, it has been conjectured that marl has been 

 produced by the detritus of other substances, and that 

 the deposit has subsided from a liquid state. This 

 supposition is strengthened by the circumstance of the 

 ■ substances occurring among the fletz, or secondary 

 strata. They are soft and opaque, and miscible with 

 water by agitation, soluble in acids with effervescence, 

 harden in the fire, and vitrify with a strong heat; and 

 to constitute true marls, the substances must contain as 

 as much clay as to fall into a powder in water, and 

 crumble into minute pieces by exposure to the air, and 

 generally showing a heavy congelation, from the effects 

 of the rays of the sun. The quantity of the calcareous 

 matter varies from two-thirds to four-fifths, and may 

 be separated by most of the acids, which will wholly dis- 

 solve the substance, and leave a residue of clay, which 

 is composed, as usual, of alumina and silica. Marls are 



often found indurated, and form " Florence marble," 

 and very like a compact limestone, and also schistose, as 

 at Monte Bolca, in Italy. Earthy and hard marls are 

 supposed to be produced by the decomposition of the 

 latter, but the two kinds do not always accompany each 

 other. 



In agriculture the marls are called stony, sandy, 

 clayey, and shell marl, according to the appearances it 

 assumes in different situations where it is found at 

 various depths under the ground. The first is usually 

 called " rotten limestone " ; it is slow and very lasting 

 in operation, and very favourable to the production of 

 grasses. " Sandy" marl is most frequent in Ireland, 

 in the pits of limestone gravel, and is called limestone 

 sand. It feels gritty, and moulders slowly ; does not 

 effervesce with acids, owing to the large quantity of sand 

 in its compofition ; and on clay lands it has very much 

 improved the texture of the soil, when liberally ap- 

 plied. "Clayey" marl is found in different colours — 

 yellow, blue, red, and brown — occasioned by the sub- 

 stances to which it has been exposed, and by the sub- 

 jacent and superincumbent formations. It contains 

 more clay than other marls, generally from sixty to 

 eighty per cent., and twenty to thirty-two of carbonate 

 of lime, and eight to ten of sand, with some signs of 

 iron ; consequently it possesses a greater power of ab- 

 sorbing and retaining moisture, the feel is soft and unc- 

 tuous, flexible like a paste, and dries and crumbles by 

 exposure. The effects are very great in improving all 

 light and thin soils, sands, gravels, and loams, as the 

 clayey base adds to the staple of their soils, and pro- 

 duces consolidation. " Shell" marl is found in places 

 that have been covered with water, and is supposed to 

 have originated from testaceous animals, being composed 

 of shells converted into calcareous earth, more or less 

 refined and pure, according to the attrition and decom- 

 position they have undergone during a long period of 

 time, and according to the quantity and quality of the 

 substances that are mixed with them by the deposition 

 of the earthy and muddy matters left by the sediment 

 of waters. This kind of marl contains more calcareous 

 matter than the others, generally more than the ordinary 

 limestones. 



Most marls effervesce in acids when fresh ; after 

 burning, the ebullition ceases. But several varieties 

 are used that show no affection by acids, and have 

 long been celebrated as manures. Clay marl effervesces 

 feebly, and hafdens in the fire, while the more calca- 

 reous sorts dissolve in powder ; and all mails are easily 

 vitrified, and crumble by exposure, according to the 

 solidity of the texture, and when burnt soon fall by the 

 attraction of moisture, and feel greasy when they contain 

 any particles of mica. Marls are generally found in a 

 moist state, especially the argillaceous sort ; they soon 

 crumble by exposure, but lime is not altered. After 

 calcination, lime falls into powder by means of air and 

 moisture, but marl suffers no change. 



The effervescence of calcareous substances in acids 

 shows the presence of the article, not the quantity ; 

 the effervescence will vary according to the strength of 

 the acid, and the compactness, penetrability, and other 

 latent qualities of the calcareous bodies themselves. 



Marl contains no salts, and the composition of it is 

 very peculiar as a carbonate of lime, 



A mixed marl has been found to contain, per cent. — 



Fine sand 36 



Clay of a soapy kind 44 



Mould 5 



Carbonate of lime 14 



Gypsum , , 1 



100 



