Freshioaier Limestone in Forfarshire, ^c. 85 



question, still precipitate tufa. Thus M.Omalius d'Halloy considers the plain of 

 Ponte Lucano, between Rome and Tivoli, as the site of a lake which has be- 

 come filled up with beds of travertino ; and the small lakes of Tartari and Sol- 

 fatarra, which still deposit tufa, as the remains of this ancient expanse of wa- 

 ters. [Journ. des Mines, torn, xxxii. p. 405.] In the same manner, nearly, 

 M. Brongniart traces the origin of the hills of travertino which bound the val- 

 ley of the E!za in Tuscany, to rivulets actually existing which still deposit tufa, 

 and Avhich, before the surface was modified by the diluvian excavation of that 

 valley, flowed out and deposited tufa on a level with the summits of those hills. 

 [Oss. Foss. 2d edit. tom. ii. p. 554.] 



In their Essay on the Geology of the Environs of Paris (p. 56), MM.Cuvier 

 and Brongniart state, that they regard the freshwater strata of that country as 

 deposits formed in an immense freshwater lake ; and they infer from the cha- 

 racter of those deposits, that the freshwater of the ancient world possessed pro- 

 perties which are not observed in that of our modern lakes : in the latter we 

 find deposits of friable mud only ; but in the lakes of ancient date there were 

 formed thick, and often very homogeneous, deposits of yellowish compact 

 limestone, of whitish marl, and of flint, which have enveloped the remains of 

 the living inhabitants of the lakes, and have converted the original substance 

 of those remains into that of the calcareous or siliceous matrix which surrounds 

 them. 



Of the four particulars here enumerated as establishing a distinction between 

 the deposits of ancient and modern lakes, no less than three are supplied by 

 the marl-loch of the Bakie ; since, 1st, we have a very compact limestone, cry- 

 stalline in parts ; 2d, extensive deposits of white and yellowish calcareous 

 marl, in which testaceous remains are only of casual occurrence ; 3d, vegeta- 

 ble remains wholly converted into limestone. The only characteristic pecu- 

 liarity to be noticed in recent lacustrine deposits, not excepting those of the 

 Bakie, wherein they fail to preserve their analogy to the ancient ones, is the 

 non -occurrence in the former of flint, either nodular, like that imbedded in 

 the travertino of the Elza, or stratiform, like that presented in the siliceous 

 and calcareo-siliceous beds of the basin of Paris. Except in thermal waters, 

 decidedly under the influence and in the proximity of volcanos, no aqueous 

 deposits of silex have been seen in the act of forming: M. Brongniart therefore 

 conjectures, with some appearance of probability, that the silex of ancient 

 freshwater formations was precipitated where we now find it, from the water 

 of thermal springs. 



On the many other analogies which the deposits of the Bakie, in common 

 with most other recent lacustrine deposits, bear to those of ancient lakes, it is 



