After the mountains of shells and other recrements of aquatic animals were elevated above the water the upper heaps of them were gradually dissolved by rains and dews and oozing through were either perfectly crystallized in smaller cavities and formed calcareous spar, or were imperfectly crystallized on the roofs of larger cavities and produced stalactes; or mixing with other undissolved shells beneath them formed marbles, which were more or less crystallized and more or less pure; or lastly, after being dissolved, the water was exhaled from them in such a manner that the external parts became solid, and forming an arch prevented the internal parts from approaching each other so near as to become solid, and thus chalk was produced. I have specimens of chalk formed at the root of several stalactites, and in their central parts; and of other stalactites which are hollow like quills from a similar cause, viz. from the external part of the stalactite hardening first by its evaporation, and thus either attracting the internal dissolved particles to the crust, or preventing them from approaching each other so as to form a solid body. Of these I saw many hanging from the arched roof of a cellar under the high street in Edinburgh. If this dissolved limestone met with vitriolic acid it was converted into alabaster, parting at the same time with its fixable air. If it met with the fluor acid it became fluor; if with the siliceous acid, flint; and when mixed with clay and sand, or either of them, acquires the name of marl. And under one or other of these forms composes a great part of the solid globe of the earth. Another mode in which limestone appears is in the form of round granulated particles, but slightly cohering together; of this kind a bed extends over Lincoln heath, perhaps twenty miles long by ten wide. The form of this calcareous sand, its angles having been rubbed off, and the flatness of its bed, evinces that that part of the country was so formed under water, the particles of sand having thus been rounded, like all other rounded pebbles. This round form of calcareous sand and of other larger pebbles is produced under water, partly by their being more or less soluble in water, and hence the angular parts become dissolved, first, by their exposing a larger surface to the action of the menstruum, and secondly, from their attrition against each other by the streams or tides, for a great length of time, successively as they were collected, and perhaps when some of them had not acquired their hardest state.