MARLS NOT DISTINGUISHABLE BY SIGHT. 809 



been found. But sometimes, and more usually in the richest marls, 

 the shells are so reduced as to be scarcely (if at all) distinguishable, 

 and the mass may appear to the eye either as a barren sand or as 

 barren clay sub-soil, according to its mechanical texture, of no 

 worth or interest whatever. The touch of muriatic, or other strong 

 atid, to the earth, first moistened by water, is the only sure test. 

 If there is shelly matter (or carbonate of lime) present, the acid 

 will produce immediate effervescence and discharge of carbonic acid 

 gas. If there is no such action, the earth is not calcareous, and 

 of no value as marl (or for ccdxing), whatever it may contain of 

 other fertilizing ingredients. 



More than a hundred species of sea-shells are found in the 

 beds of marl which I have worked. Generally the shells, 

 though very fragile, are entire, though much broken by the dig- 

 ging and after-operations. The white shells are rapidly reduced, 

 after being mixed with an acid soil ; but some gray kinds, as 

 the scallop (pecten) and the oyster, are so hard as to be very long 

 before they can act as manure. Some beds, and they are gene- 

 rally the richest, have scarcely any whole shells, but are formed 

 principally of small broken fragments. Of course the value 

 of marl as a manure depends in some measure on which kinds 

 of shells are most numerous, and their state of division, as 

 well as upon the total amount of the calcareous earth contained. 

 The last is, however, by far the most important criterion of 

 value. The most experienced eye may be much deceived in the 

 strength of marl ; and 'still more gross and dangerous errors 

 would be made by an inexperienced marler. The strength of a 

 body of marl often changes materially in sinking a foot in depth 

 although the same changes may be expected to occur very regu- 

 larly, in every pit sunk through the same bed. The annexed 

 figure will serve better to illustrate both these changes in perpen- 

 dicular extension in a marl-bed, .and the regularity of quality in 

 horizontal extension. 



Such as this is no uncommon character of a bed of marl, and 

 such I have worked, and could recognise the identity of the 

 several layers, by their appearance, in different diggings, half a 

 mile or more apart. Thus, suppose the two ends of the section 



