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AS AMANURE.| 579

there are fome exceptions that ought to be taken notice of,

iff, There are fome kinds of lime-ftone, that, when analyfed, are found to contain a refiduum confifting of a foft flimy-like fubftance. This is always in very fmall pro-= portions, and has probably been formed by a fediment fubfiding from the water while the rock was forming. It feems proba- bly, that this kind of lime would be lefs proper to be employed as a cement than as a manure.

2d, Although marle and lime-ftone are juftly enough diftinguifhed in the text(§ E p. 402); yetit happens, that clay and fand are found naturally mixed with one another, in fuch various proportions, and in thefe {tates joined with calcareous matter, that there is no poffibility of afcertaining the exact point where marle ends, and lime- {tone begins.

A very{mall proportion of clay is fuffici- ent to make an exceedingly hard lime-itone relent in time in the air, and fall to pieces; fo that there are many{forts of{tone marle that confift chiefly of fand and calcareous earth, and only a very little clay.

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