448 LOSS OP CALCAREOUS MATTER. 



ing matter. The continued purity of this water is an additional 

 proof that the blue colouring matter is chemically combined with 

 the carbonate of lime and the combination is a visible illustra- 

 tion of the manner in which marl holds to and fixes putrescent 

 manures. 



Mr. William Carmichael, of Queen Ann's county, Maryland, an 

 intelligent agriculturist, and an experienced and observant marler, 

 is of opinion that there is a perceptible superiority of effect of blue 

 marls over others of equal (and even greater) strength in calcareous 

 matter. (Farmers' Register, vol. vii. p. 106.) This superiority 

 of effect probably is caused by-the vegetable or other putrescent and 

 alimentary matter being combined with the calcareous, and by its 

 presence giving colour to the blue marl. And that the blue colour 

 is thus produced is fully proved by the facts stated at page 445, 

 and by my more general observation. 



Excepting then the additional value in the vegetable extract 

 which gives the colour, there is no difference between the blue and 

 the yellow marls, other than the difference, as of any marls of simi- 

 lar colour, in their respective amounts of calcareous matter. And 

 the same may be said of wet and dry marls, which are generally, 

 but not always, distinguished by the above colours ; and also of any 

 other miocene marls, excepting for such small proportion of " green- 

 sand" as is sometimes present. But there is reason to believe that 

 wet marls, in many cases, have lost some of their ancient strength, 

 by the continued though very slow percolation and subsequent dis- 

 charge of water through the mass. If recent rain-water penetrates 

 wet marl, it dissolves some carbonate of lirne (by means of the car- 

 bonic acid in the rain-water) ; and, as the water slowly flows off, or 

 oozes out, instead of being evaporated, the dissolved lime is washed 

 into the nearest stream, and is lost, instead of being left, crystal- 

 lized or otherwise, as in dry marl. Again if water flows over 

 having sulphate of iron (copperas) in solution, (which is not a very 

 rare case,) that dissolved salt acts with the carbonate of lime to 

 produce the decomposition of both the sulphate of iron and the 

 carbonate of lime, and from two of their component parts to form 

 sulphate of lime. And as this is slightly soluble in water, it must 

 be carried off by the slowly oozing water, as long as any of these 

 new salts remain. In this case, the carbonic acid is evolved, and 

 the iron is precipitated and often fills, or coats the interior of the 

 spaces before filled by the shells which this chemical process had 

 decomposed and removed. This effect, when produced, is seen at 

 the upper part of the marl, where the copperas water first touches 

 the shelly matter. LrHenrico, near the western limit of the marl, 

 and in Hanover, more eastward, there is generally over the pre- 

 sent highest shells a body of earth of colour and general appear- 

 ance very similar to the marl below, and full of hollow impressions 



