-----+------------+-----------+-----------+-------------
| | Barbadoes. | Luneburg. | Ayrshire. | Wesermarsh.
+---------------------------+------------+-----------+-----------+-------------
| Carbonate of lime | 93.2 | 85.4 | 8.4 | 8.2
| Carbonate of magnesia | ... | 1.3 | ... | 3.0
| Sulphate of lime | ... | 0.1 | ... | 0.5
| Phosphate of lime | 0.1 | 2.3 | ... | 1.2
| Alumina and oxide of iron | 1.6 | 4.6 | 2.2 | 7.2
| Alkaline salts | ... | 0.1 | ... | 1.0
| Silica and clay | 4.6 | 5.6 | 84.9 | 78.9
| Organic matter | 0.5 | 0.6 | 2.8 | ...
| Water | ... | ... | 1.4 | ...
+---------------------------+------------+-----------+-----------+-------------
| | 100.00 | 100.00 | 99.7 | 100.00
+---------------------------+------------+-----------+-----------+-------------

The true marls, that is those in which carbonate of lime abounds, are
greatly preferable to clay marls, the latter, indeed, operate chiefly
mechanically, by altering the texture of the soil--the lime they contain
being frequently too small to exercise much appreciable effect.

Shell and coral sands consist chiefly of fragments of shells and coral
disintegrated by the action of the waves, and mixed with more or less
siliceous sand, and containing small quantities of phosphate of lime.
They occur to a considerable extent both on our own coasts and those of
France, and have been used with good effect on some descriptions of
soil.

The general composition of limestones has been already adverted to,
when treating of the origin of soils, and a distinction drawn between
the common limestones and dolomite or magnesium limestone. Few
limestones can be considered as even approaching to purity, and they
almost all contain a small quantity of carbonate of magnesia as well as
earthy matters, and occasionally a little phosphate of lime. In good
specimens the quantities of these substances are generally small, and
they usually contain about half their weight of lime. When limestone is
burnt in the kiln, the change which ensues consists in the expulsion of
the carbonic acid, and the consequent conversion of the lime into the
uncombined or