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ciently near the surface to be easily and economically uncovered. 

 A rather sure guide to the marl is afforded by the aspect and 

 composition of the earth existing at and near the surface. Should 

 it be at all greenish, or contain upon close inspection, any of the 

 green granules, the probability is that the marl lies beneath at a 

 very moderate depth, and the chances augment when we find our 

 borings bring up an increasing proportion of this mineral as we 

 descend. 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 almost 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 fami- 

 liarity with the multiform aspects which it assumes is indispen- 

 sable. 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 most, or sometimes all of it; and 

 that it seems, moreover, to be dependent upon the proportion in 

 these green grains of those powerful alkaline stimulants to vege- 

 tation, potash and lime, but especially potash. The first thing 

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

 in the whole mass, and this may be effected v.ith a greater or 

 less degree of accuracy in several ways. The simplest and 

 readiest method is to employ a small pocket magnifying-glass, 

 and to become familiar with the dark-green grains, so as to dis- 

 tinguish them at once from other dark varieties of sand which 

 sometimes occur associated 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 of the green 

 granules. But as these granules cannot sometimes be distin- 

 guished 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 

 adopt some method of bringing out under the magnifier their dif- 

 ferent characteristics of colour and form. Let the mass be washed 

 in a large glass 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 rins- 

 ings, and permitting it to settle until clear, we may estimate the 

 comparative quantity of clay in different marls by the relative 



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