TRAVERTINE. 



565 



possible that the lime which enabled the mollusca to form their shells, was supplied by 

 calcareous springs, though it is more probable that the minute quantity required for 

 the wants of fluviatile testacea, may have been afforded by the percolation of rain water 

 through the surrounding rocks. Fully to comprehend the method in which such beds 

 of shell marl have been formed, the reader must consult the writings of Mr. Lyell, who 

 has so admirably described deposits of this nature in the lochs of Forfarshire, North 

 Britain. (See Geol. Trans., 2nd series, vol. ii. p. 72.) The Montgomeryshire marl agrees 

 with that of Forfar, in being covered by a thick stratum of black peat. 



As this marl of Sutton is an excellent manure, and the surrounding country contains 

 no limestone, its discovery is of considerable practical importance ; for although the 

 hollow in which it is deposited is small, analogy may fairly lead us to infer, that similar 

 and larger basins of the same substance, may yet be discovered beneath the turbaries 

 and mosses so numerous in the upland valleys of Montgomeryshire. 



Travertine. 



We have now reached the last stage of our researches into the nature of the super- 

 ficial deposits and have only to explain the production of those modern rocks, formed 

 under the atmosphere, and to which the name of travertine has been applied. Such 

 rocks have been deposited from the remotest periods of history to the present day, 

 and in most countries. They are very abundant in districts which have been or still 

 are subjected to volcanic action, as in the environs of the Tiber, whence their name of 

 travertine 1 . They are, however, almost equally abundant in countries, where there are 

 no distinct appearances of volcanic action near the surface. All that seems necessary 

 for their production is, that a spring should hold in solution a sufficient quantity of car- 

 bonate of lime, which, upon the escape of the excess of carbonic acid gas into the atmo- 

 sphere, deposits its earthy residuum, thus encrusting the sticks, leaves, shells, or any 

 other object over which the water flows. 



There are many such springs in different parts of the region laid down in the an- 

 nexed map, but from the insignificance of their deposits, few are worthy of attention. 

 The strongest of them issue from the central member of the Old Red System, which 

 is chiefly composed of marls and cornstone (concretionary limestone). A good example 

 occurs two miles north of Ludlow, on the property of Sir W. Boughton, Bart., where a 

 brook forms large and solid masses on its sides. The most remarkable, however, of 

 these deposits are in the deep dingles called Piper's Brook, Sapey Brook, and at South- 

 stone Rock, on the right bank of the Teme, between Tenbury and Knightwick Bridge. 

 In Piper's Brook it partially encrusts the sides of the stream, and is found in large 

 masses rolled down its bed. 



1 "Lapis Tiburtinus." — Ital. Travertino ; Eng. Travertine. 



4 B 



