Calcareous Springs. Rain Water, containing carbonic acid gas, and other waters also more highly charged with this gas, have the property of dissolving the carbonate of lime with which they come in contact, as they percolate through the strata of rock beneath the surface. When the water rises in springs, it comes charged with calcareous matter; and as it evaporates, this load is deposited in the form of calcareous incrustations. Such springs sometimes rise through granitic rocks and other formations, which contain little or no limestone, this being in these instances supplied to the water from some distant formations through which it has flowed. Thus the carbonate of lime required by shell fish and plants is distributed abundantly in places that would otherwise be destitute of it. Sir Charles Lyell states that in central France, a district where the primary rocks are unusually destitute of limestone, springs copiously charged with carbonate of lime rise up through the granite and gneiss. Some of these are thermal, and probably derive their origin from the deep source of volcanic heat once so active in that region.

One of these springs near'Clermont has formed by its incrustations an elevated mound of travertine, or white concretionary limestone, 240 ft. in length, and at its termination 16 ft. high and 12 ft. wide. Another in the same region rises in a gneiss country at the foot of a volcanic cone, at least 20 m. from any calcareous rock. The deposit'of these springs is often a spongy, porous substance called calcareous tufa, or calc tuff. It takes the impression of the objects it encloses, as leaves, twigs, and branches of trees, and retains the forms, if not the material itself, in its solid substance. When freshly quarried, it is easily cut into any shape, and is therefore conveniently applied to building purposes. The temples of Psestum are built of it, and the stone has in them assumed great strength and solidity. In the central parts of New York, especially in the vicinity of Seneca and Cayuga lakes, deposits of this nature are very frequent. They form beds of marl beneath muck swamps, and in the bottoms of ponds and lakes. Wherever the calcareous water flows, the aquatic plant chara grows abundantly, so as sometimes to obstruct the watercourses, and render its removal necessary.

As the plant grows, its stems become incrust-ed with carbonate of lime, and new green growth continues to shoot out beyond, which is soon to be filled in with the same stony incrustation. The abundance of calcareous matter is as favorable to the growth of freshwater testacea as of the chara; and those which are found in the oldest of these formations are still of the common living fresh-water species.