In many places the value of land has been much augmented by the application of marl. Treating of this article in a practical way, it may be divided into shell-marl and earth-marl. Shell-marl is composed of animal shells dissolved; earth-marl is also fossil. The color of the latter is various, its hardness being sometimes soft and ductile, like clay; sometimes hard and solid, like stone; and sometimes it is extended into thin beds, like slate. Shellmarl is easily distinguished by the shells, which always appear in it; but the similarity betwixt earth-marl and many other fossil substances, renders it difficult to distinguish them.
Shell-marl is very different in its nature from clayey and stone marls, and, from its effects upon the soil, is commonly classed among the animal manures: it does not dissolve with water as the other marls do. It sucks it up, and swells with it like a sponge. dr. Home says, that it takes six times more of acids to saturate it than any of the other marls which he had met with. But the greatest difference betwixt the shell-marl and the other marls consists in this, the shell-marl contains oils.
This marl, it would seem from the qualities which it possesses, promotes vegetation in all the different ways. It increases the food of plants; it communicates to the soil a power of attracting this food from the air; it enlarges the pasture of plants; and it prepares the vegetable food for entering their roots.
See also: List of minerals