MANURES. 55 



occur, but where a covering of any of the superficial deposits obscures it, the 

 primary point to be remembered is, that the true marl is the lowest deposit of 

 the region. We should find out, therefore, the deepest impressions of the land, 

 as the meadows and natural ravines, and by the use of an auger or other in- 

 strument for probing the ground, several feet in length, we may very frequently 

 ascertain whether the stratum lies sufficiently near the surface to be easily and 

 economically uncovered. A pretty sure guide to the marl may be found in. 

 some places from the aspect and composition of the earth upon and near the 

 surface. Should it be at all greenish, or contain, on close inspection, any of 

 the green granules, the probability is pretty strong that the marl lies beneath, 

 at a very moderate depth, and the likelihood of this is augmented when we find 

 our borings bring up an increasing proportion of this mineral as we descend 

 deeper. I have repeatedly found the position of the marl stratum indicated 

 by the trickling forth of the water from the foot of a bank, for the water is al- 

 most invariably seen to issue along the top of either the dark clay or the true 

 marl. 



For judging of the quality of a marl by observation, some familiarity with 

 the multiform aspects which it puts on is indispensable. The leading rule, 

 however, is, to bear in mind that the fertilizing efficacy of the compound, 

 resides in the minute round greenish grains which compose more or less of it, 

 or sometimes all of it, and that it seems, moreover, to be dependant upon the 

 proportion which these green grains contain of those powerful alkaline stimu- 

 lants to vegetation, potash and lime, but more especially potash. The first thing, 

 then, is to approximate to the relative quantity of the green grains in the whole 

 mass, and this may he effected, to a greater or less degree of accuracy, in seve- 

 ral ways. The simplest and surest method is to employ a small pocket mag- 

 nifying-glass (a common burning-glass will answer,) and to make the eye 

 familiar as soon as possible with the dark green grains, so as to distinguish 

 them at once from other dark varieties of sand which sometimes occur asso- 

 ciated with them. A little practice will very soon enable one to use the glass 

 expertly, and to arrive at a pretty true estimate of the probable per centage in 

 which the green grains may exist. But as these granules cannot, sometimes 

 be distinguished from the grains of ordinary white flinty sand, or from other 

 kinds, in consequence of the particles being all alike coated with a thin film of 

 the dark cementing clay, it will be useful to devise some method of bringing 

 out under the magnifier their different characteristic traits of colour and form. 

 Let the mass be washed in a large slass tumbler, and repeatedly agitated with 

 the water, until as much of the clay as possible has been detached from the 

 grains. After pouring off the turbid water by repeated rinsings, and permit- 

 ting it to settle until clear, we may estimate the comparative quantity of clay 

 in different marls by the relative amount of sediment which subsides. If we 

 wish to be more accurate, we can weigh out a given quantity of the marl, then 

 pursue the above plan, and decant the clear water from the clay, and after 

 thoroughly drying the clay, weigh it to ascertain its amount. Having got 

 away most of the clay, we should spread out the granular matter upon a sheet 

 of paper and dry it, when there will be no farther difficulty in distinguishing, 

 by their colour and lustre, the foreign impurities, from the grains of the true 

 marl, and also of estimating the relative abundance of each. When the marl 

 to be examined contains much clay. I would recommend the experiments to 

 be made upon a regularly weighed quantity, weighing both the clayey and the 

 granular portions. A delicate apothecary's balance will commonly be found 

 accurate enough. Another more expeditious, though less accurate method, is 

 merely to dry the marl, and spread it extremely thin upon a sheet of white 

 paper, and then to hold it near a window or in the light, and to examine it 

 carefully by the magnifier. The flinty sand, though stained with clay, may 

 then be clearly discerned, in consequence of its transparency, whereas when 

 we inspect a solid lump, all the particles upon the surface are nearly alike 

 dark. 



A good suggestion is, to place a portion of the marl upon a hot shovel or on 

 the top of a stove, when all the marl grains will change from their ordinary 

 green to a light red or brick colour, while the other materials of the mass sus- 

 tain little alteration. This will often make it obvious to the naked eye in what 

 proportion the green grains are present. 



