84 Mr. Lyell on a recent Formation of 



being principally characterized by the cavities which traverse its substance. 

 " These cavities/' he observes, "are of two kinds : the former, small, length- 

 ened, dull in the interior, and often inclosing vegetable remains, by which 

 they seem to have been occasioned ; the latter, very long, passing from the 

 top to the bottom, and so parallel to one another as to appear artificial." 

 [Geog. Beobach. band. 2. seite 20.] These cavities are considered by 

 M. Brongniart as of invariable occurrence in the freshwater limestones of 

 all countries, and as absolutely characteristic of such formations. [Oss. 

 Foss. 2d edit. torn. ii. p. 548.] Similar cavities were noticed by M. Omalius 

 d'Halloy in the travertino near Cistern e, at the entrance of the Pontine Marshes, 

 on the road from Rome to Naples ; and he also observed the occurrence of 

 LymneaB and Helices in the rock. [Journ. des Mines, torn, xxxii. p. 402.] The 

 freshwater formation of the valley of the Elza, in Tuscany, towards Colle, is 

 described by M. Brongniart as presenting, 1st, upon the heights, as you de- 

 scend towards the valley, a very compact limestone of great extent and thick- 

 ness : 2d, at a lower level, a fine sand, inclosing Neritffi which retain their co- 

 lour, and a multitude of other freshwater shells : 3d, on descending the valley, 

 a stratum, which consists either of compact limestone traversed by sinuous ca- 

 vities, and containing freshwater shells and flinty portions that gradually pass 

 into the matrix ; or of a porous rock resembling tufa, composed apparently of 

 the stems of Chara and Myriophyllum converted into limestone. [Oss. Poss. 

 2d edit. tom. ii. p. 554.] 



There is scarcely more than one circumstance in this description (and to 

 that one I shall recur in the sequel) to which a parallel cannot be found in the 

 deposits of the Bakie. Nor ought the resemblance which the ancient traver- 

 tino bears, in common with modern tufa, to the rock-marl of the Bakie, to be 

 thought surprising, when we consider what the origin of the former has been, 

 in the opinion of the most able geologists. It is regarded by them as a deposit 

 which has been formed, at periods more or less remote*, in the natural hol- 

 lows of lakes ; the extent of those lakes being in some cases yet assignable ; 

 and the supply of the calcareous matter which formed the deposit, has been 

 traced by them to springs or streams which existing in the very districts in 



* MM. Brongniart and Omalius d'Halloy, who have each examined the travertino of theCam- 

 pagna, differ considerably in their estimates of its antiquity. The one, not finding on its surface 

 any marks of diluvian action, considers it as of post-diluvian formation [Journ. des Mines, 

 tom. xxxii. p. 405.] : the other, referring, it should seem, to one epoch the travertinos of the 

 Campagna and of Tuscany, must regard that epoch as ante-diluvian ; since the surface of the 

 Tuscan travertino has been greatly modified by diluvial excavation. [Oss. Foss. 2d edit. tom. ii. 

 pp. 531, 554.] 



