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disproportionately small, and the matter of the shells often so firm 

 and unsusceptible of that easy disintegration, necessary to form a 

 calcareous marl adapted to act speedily upon the crop, that the 

 striking effects witnessed from the marl can in nowise be attributed 

 to the trivial amount of lime which the shells may occasionally fur- 

 nish to the land. Nevertheless, as some feebly beneficial efl^ects may 

 possibly arise from this source, it may be of service to the agri- 

 culturist, in choosing between different fossiliferous marls, to at- 

 tend to the nature of the particular fossils, and the state of more 

 or less decomposition or change in which they are to be found. 

 It must be borne in mind, that a large portion of the visible marl 

 stratum is immediately overspread by a very porous layer of yel- 

 low ferruginous sand, and that this introduces to it a perpetual 

 supply of water, furnished with great regularity as from an im- 

 mense filter. From the upper or f err v gin ous sand it must descend 

 charged often with a considerable amount of the oxide of iron, 

 as may be seen in the abundant ochreous sediment which it almost 

 always deposits as it issues from the surface or upper part of the 

 marl bed. It is ready, therefore, to precipitate this oxide of iron 

 upon any substance capable of displacing it from the water, and 

 meeting with the more soluble carbonate of lime of the shells, an 

 interchange of materials arises, and the calcareous matter of the 

 shells is dissolved and carried away, while the oxide of iron takes 

 its place. Hence we often see the shells of a deep yellow or brown 

 colour, and upon inspection they are found to consist less of car- 

 bonate of lime than of oxide of iron. In such case they are to 

 be regarded as wholly inert upon the soil, as, in fact, so much 

 useless matter, usurping the place of a far more serviceable sub- 

 stance, the greensand or marl. But this is not the only change 

 which seems to have been effected in the foreign materials of the 

 marl by this unceasing infiltration of water. I have before alluded 

 to the peculiar composition of the overlying dark-blue astringent 

 clay, and to the fact that it frequently contains a sensible quantity 

 of the sulphate of iron or copperas; and that both this clay and 

 its astringent impregnations are very often mingled through the 

 granular marl itself Now the water from either of these sources 

 must dissolve in its passage a considerable quantity of the cop- 

 peras (an easily soluble substance), and where there are shells or 

 other calcareous fossils, it must carry with it a portion of the car- 



