The formation of Travertine is going on with no less activity in other parts of the Italian Peninsula. At the baths of San Filippo, in Tuscany, there are three warm springs which contain a very large amount of mineral matter in solution. The water which supplies the baths falls into a pond, where it has been known to deposit a solid stratum of rock _thirty feet thick_ in twenty years. In the same neighborhood are the mineral baths of San Vignone. The source from which the water flows is situated on the summit of a hill not more than a few hundred yards from the high road between Sienna and Rome; and so rapid is the formation of stone, that half a foot of solid Travertine is deposited every year in the pipe that conducts the water to the baths. At this spot we have a very good illustration of the argument we are now considering. As the stream of water flows down the slopes of the hill, a thin layer of Travertine rock is produced on the surface of the earth, almost before our eyes; and so it was previous to our own time, and so it has been for ages, as history and tradition testify. The quantity produced in each year and in each century is comparatively small, but we can have no doubt that it _has_ been produced by the means described. Now, beneath the surface of the Earth, immediately below these modern formations, of which we have so clearly ascertained the origin, we find strata of the same kind, composed of the same materials, and arranged in the same way, layer resting upon layer, down to a depth of two hundred feet: and the Geologist accounts for the formation of the one according to the same laws which he has seen at work in the production of the other.[48] [Illustration] [Illustration] _CHAPTER VIII._ STRATIFIED ROCKS OF ORGANIC ORIGIN--ILLUSTRATIONS FROM ANIMAL LIFE. _Nature of organic rocks--Carbonate of lime extracted from the sea by the intervention of minute animalcules--Chalk rock--Its vast extent--Supposed to be of organic origin--A stratum of the same kind now growing up on the floor of the Atlantic ocean--Coral reefs and islands--Their general appearance--Their geographical distribution--Their organic origin--Structure of the zoophyte--Various illustrations--Agency of the zoophyte in the construction of coral rock--How the sunken reef is converted into an island and peopled with plants and animals--Difficulty proposed and considered--Hypothesis of Mr. Darwin--Coral limestone in the solid crust of the earth._