83 Mr! Lyell on a recent Formation of 



tion of the water of the springs on the previously existing shell-marl. — I have 

 already stated that it is only near the springs that the rock-marl is found ; that 

 where it occurs^ shell-marl has always hitherto been found beneath it^ and 

 that the water of the springs contains carbonic acid. May we not then rea- 

 sonably suppose that this carbonated water, in percolating the lowest deposits 

 of shell-marl, took up from them a portion of lime; and on mixing with the 

 water of the lake at a further distance from the source, let fall a calcareous pre- 

 cipitate, which in some places falling alone, produced a pure crystalline lime- 

 stone, and in others, mixing with the later deposits of shell-marl and the va- 

 rious lacustrine plants, had the effect of cementing them together into one 

 tufaceous mass. 



The only other instance that has come to my knowledge in this part of 

 Scotland of a limestone similar in character and in the circumstances of its 

 occurrence to that of the Bakie, is in the moss of Cookstone, near the Bakie, 

 and like that loch, occupying one of the hollows between hillocks of diluvium. 

 For this piece of information I am indebted to Mr. Blackadder of Glammis, a 

 gentleman well skilled in every branch of geological research. An inconsider- 

 able quantity was discovered in cutting peat ; but nothing further can at pre- 

 sent be ascertained respecting it. If a similar rot:k has been found in any 

 other marl-loch, it is probably in very small quantity ; for marl is so extensively 

 worked in this part of Scotland, that were there any considerable mass of the 

 substance in question, it could scarcely have escaped observation. A forma- 

 tion, however, is now daily going on in the Loch of Forfar, which, though not 

 strictly analogous to that of the Bakie and of Cookstone, may still tend to 

 throw some light on the subject. For the first intimation of the existence of 

 this formation I am also indebted to Mr. Blackadder. 



Tufa in the Loch of Forfar. — The Loch of Forfar was never more than 

 partially drained ; and the marl can only be procured there by dredging it 

 from the bottom. When the level of the lake was first lowered, a bed of shell- 

 marl of very pure quality was left dry on its southern bank, some of which, 

 being too thin to be worth working, still remains there. It has been since bu- 

 ried by the falling in of some of the diluvial gravel of the bank above, part of 

 which has also covered the bottom of the lake near the shore. The water, 

 when the lake is very full, or the wind high, washes the edge of the bed 

 of marl in the bank, and becomes milky from the quantity of calcareous mat- 

 ter suspended in it. From this fluid a deposit is constantly taking place, and 

 a thin crust of calcareous tufa, which scarcely ever attains the thickness of an 

 inch and a half, falling on the gravel, cements it together, and forms a breccia. 

 It occasionally incloses a few shells, chiefly of the Helix peregra. These have 



