Hard water flowing in a surface stream, if dashed into spray at a cascade, may deposit its lime carbonate in an ever thickening veneer wherever the spray is dashed about the falls. This material, when cut in section, has waving parallel layers and is known as _travertine_ or _calcareous sinter_. Some of the most remarkable deposits of this nature may be seen at the cascade of Tivoli near Rome, and most of the Roman buildings have been constructed from travertine that has been quarried in the vicinity. =The growth of stalactites.=—Water, after percolating slowly through the crevices of limestone, where it becomes charged with the carbonic acid gas and with dissolved carbonate of lime, may trickle from the roof of a cavern. Emerging from the narrow crevice, it may give off some of its contained gas and is usually subject to evaporation, with the result that the lime carbonate is left adhering to the rock surface from which evaporation took place. If the water collects upon the cavern roof so slowly that it can entirely evaporate before a drop can form, the entire content of carbonate will be left adhering to the roof. Evaporation is most rapid near the margins and over the center of each drop as it develops, and the deposit which is left thus takes the form of tiny white rings at those points upon the crevice where there is the easiest passage for the trickling water. To the outer surface of these rings water will first adhere and then evaporate, as it will also slowly ooze through the passage in the ring, but here without evaporation until it reaches the lower surface. A pendant structure will, therefore, develop, growing outward in all directions by the deposition of concentric layers which are thickest near the roof, and downward into the form of a rock “icicle” through evaporation of the water which collects near the tip. These pendant sinter formations are known as stalactites and are thus formed of concentric layers arranged like a series of nested cornucopias with a perforation of nearly uniform caliber along the axis of the structure (Fig. 194). [Illustration: FIG. 194.—Diagrams to show the manner of formation of stalactites, stalagmites, and sinter columns beneath parallel crevices upon the roofs of caverns (in part after von Knebel).]