Calcareous Springs

Calcareous Springs occur mostly in limestone districts, especially along the outcrop of the junction of the limestone with underlying impermeable beds. The water, even if only slightly impregnated with the soluble calcium-bicarbonate, on coming to the surface parts with some of its carbon-dioxide, and consequently calcium-carbonate, which is insoluble in pure water, is precipitated. This parting with carbon-dioxide may sometimes arise merely from evaporation; but it seems mostly due to the action of living green aquatic plants, such as Chara, mosses, and such flowering-plants as Ranunculi and Potamogeton, which take in and decompose this gas. The limestone is accordingly deposited upon the plants, and the springs, though in truth merely encrusting, are popularly called "petrifying." The precipitated limestone, known as calc-tuff, calc-sinter, or travertine (q.v.), may form a compact building-stone, and sometimes accumulates with great rapidity, as at San Filippo in Tuscany, where deposits three feet thick are formed in a year.