DNLIKE EFFECTS OF DIFFERENT MARLS. 371 



The characteristic property of true marls of every variety is, I have 

 said, the presence of a considerable per centage of carbonate of lime in 

 the state of a fine powder, and, in general, diffused uniformly through 

 the entire mass. To this calcareous matter the chief efficacy of these 

 marls is no doubt to be ascribed, yet as they always contain other chem- 

 ical compounds to which the special efficacy of certain varieties has 

 sometimes been ascribed, it may not be improper to direct your attention 

 to the preceding table, in which the constitution of several marls, from 

 different localities, is represented, after the analyses ot Sprengel. 



Several reflections will occur to you on looking at these tables — such 

 as. 



First — that marls differ very much in composition, and therefore 

 must differ very much also in the effects which they are capable of pro- 

 ducing when applied in the same quantity to the same kinds of land. 



Seco7id — that, among other differences, the proportion of carbonate 

 of hme is very unlike — in so Tie varieties amounting to 85 lbs. out of 

 every hundred, while in others ^s little as 5 lbs. are present in the same 

 weight. You will understand, . ^erefore, how very different the quan- 

 tity applied to the land must be, it . "ese several varieties are to produce 

 an equal liming or to add equal quantities of lime to the soil. You 

 will see that each of three persons may be adopting the best practice 

 with his own marl— though the one add only 12 to 20 tons per acre, 

 while the second adds 50 to CO, and the third 100 to 120 tons. 



Third — that the proportion of phosphate of lime (bone-earth) is in 

 some marls considerably greater than in other's. Thus with every ton 

 of the first of the above marls you would lay on the soil 52 lbs. of bone 

 earth — about as much as is contained in a cwt. of bone dust — while 

 with the second you would only add 11 lbs. In so far as their effects 

 upon the land depend, or are influenced by the presence of this sub- 

 stance, therefore, they must also be very different. And, 



Fourth — that the mechanical effects of these marls upon the soil to 

 which they are added must be very unlike, since some contain 70 or 

 SO lbs. of sand in every hundred — while others contain a considerable 

 quantity of clay. The opening effects of the one marl, and the stiff- 

 ening effects of the other, when they are laid on in large quantities, 

 cannot fail to produce very different alterations in the physical cha- 

 racters of the soil. 



2^. Shell Sand.— The sands that skirt the shores of the sea are 

 found in many localities to be composed, in large proportion, of the 

 fragments of broken and comminuted shells. These form a calcare- 

 ous sand, mixed occasionally with portions of animal matter, and, 

 when taken fresh from the sea-shore, with some saline matter derived 

 from the sea. 



Such is the case in many places on the coast of Cornwall. From 

 these spots the sand is transported to a distance of many miles into 

 the interior for the purpose of being laid upon the land. It has been 

 estimated (De la Beche's Geological Report on Cornwall^ ^c, p. 480) 

 that seven millions of cubic feet are at present employed every year 

 in that county for this purpose. 



On the western coast of Scotland also, and on the shores of the island 

 of Arran and of the Western Isles, this shell sand abounds, and is 

 16* 



