THE FARMER'S MAGAZINE. 



383 



fresh-water denomination. Marls have been found 

 stratified in some parts of Europe. 



The component parts of marl are so minutely 

 divided as to be invisible to the naked eye, and 

 from this circumstance, and from their containing 

 both fresh and salt water organic remains, and 

 from the fissile structure, it has been conjectured 

 that they have been produced from the detritus of 

 other substances, and that they have subsided from 

 a liquid state. This su|)position is strengthened 

 by the circumstance of the substances occurring 

 among the secondary strata. They are soft 

 and opaque, earthy, light, and miscible with 

 water by agitation, soluble in acids with effer- 

 vescence, hard in the fire, and vitrify with a strong 

 heat ; and to constitute true marls, the substance 

 must contain as much clay as to faff into a powder 

 in water, and crumble into minute pieces by ex- 

 posure to the air, and generally showing a hoary 

 congelation from the effects of the rays of the sun. 

 The quantity of calcareous matter varies from two- 

 thirds to four-fifths, which may be separated by 

 most of the acids, which will wholly dissolve the 

 substance and leave a residue of clay, which is 

 composed, as usual, of alumina and silica. Marls 

 are often indurated, and form " Florence marble," 

 and very like a compact limestone ; and also 

 schistose, as at Monte Bolea, 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 

 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 composi- 

 tion, and on clayey lands it has very much improved 

 the texture of the soil when liberally applied. 

 " Clayey marl " is found of different colours, yellow, 

 blue, red, and brown, occasioned by the substances 

 to which it has been exposed, and by the subjacent 

 and superincumbent form.ations : it contains more 

 clay than other marls, generally from 60 to 80 per 

 cent., and 20 to 32 of carbonate of lime, aud 8 to 

 10 of sand, with some signs of iron ; consequently 

 it possesses a greater power of absorbing and re- 

 taining moisture : the feel is soft and unctuous, 

 fiexible like a paste, and dries and crumbles by ex- 

 posure. The effects are very great in improving all 

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

 the clayey basis adds to the staple of the thin soils 

 and produces consoUdation. " SheU 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 decomposition they 

 have undergone during a long period of time, and 

 according to the quantity and quality of the sub- 

 stances that are mixed with them by the deposition 

 of the early and muddy matters left by the sediment 



of the 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 tliat show no affection by acids, 

 and have been long celebrated as manures. Clay 

 marl effervesces feebly and hardens in the fire, 

 while the more calcareous sorts dissolve in powder, 

 and all marls are easily vitrified, and crumble by 

 exposure according to the sohdity of the texture, 

 and when burnt soon fall by the attraction of mois- 

 ture, and feel greasy when they contain any parti- 

 cles of mica. Marls are generally found in a moist 

 state, especially the argillaceous sort ; they soon 

 crumble l)y exposure, but lime is not altered. 

 After calcination, lime falls into powder by means 

 of air and water, but marl suffers no change. 



The efferv^escence of calcareous substances in 

 acids shows the presence of the substance, not 

 the quantity ; the effervescence will vary according 

 to the strength of the acid, and the compactness, 

 penetrability, and other latent qualities of the cal- 

 careous bodies themselves. Marl contains no salts, 

 and the composition of it is very peculiar as a car- 

 bonate of lime. 



A mixed marl has been found to contain : — 



Fine sand - 36 



Clay of a soapy kind 44 



Mould 5 



Carbonate of lime 14 



Gypsum 1 



100 

 The chalk marl in Norfolk contains in 100 

 grains : — 



Chalk 85 grains. 



Sand 10 „ 



Clay 5 „ 



100 „ 

 Clay marl of the same county in 100 grains 

 contains : — 



Clay with some iron 50 



Impure chalk 43 



Sand 7 



100 

 A kind called " Dove marl," from the similarity 

 to pigeon's dung, contains as much as 98 per cent, 

 of carbonate of lime. 



Marls have been supposed to be derived from 

 the ruins of the primary and secondary rocks, worn 

 down, carried about, agitated and deposited without 

 any relation to the laws of specific gravity. Animal 

 remains are found at considerable depths, and even 

 stones of great weight are met with, where no rocks 

 of the same or of similar kind are knov/n to exist 

 in the surrounding locality or in the adjacent geo- 

 logical formations. Fire wholly changes the nature 

 of bodies that are subjected to the violent nature of 

 its influence, and gives them qualities they did not 

 before possess, and banishes others which they 

 never afterwards recover. Decomposed lavas are 

 exceedingly fruitful, and the heat of volcanoes pro- 

 duces a most luxuriant vegetation in places within 



