178 GEOLOGY. 



The present configuration of the land at Blashenwell puts the 

 possibility of a lake, out of the question: a gradual declination 

 from the spring to the clays near the gate, proves that a change 

 of level must have taken place, causing the destruction of this 

 ancient lake, of which no trace remains visible except by a care- 

 ful examination of the site of the hypothetical lake. 



I now come to the last portion of my rejoinder, with regard to 

 my assertion that the deposition is mechanical, and not chemical. 

 I merely made use of the term incidentally, without intending 

 to convey to the reader that I was prepared to go into the 

 scientific cause of the deposit. It is well known, that if lime 

 is added to water already charged with lime, the carbonate 

 becomes a bi-carbonate. Under these circumstances, it is not 

 unreasonable to suppose that during the introduction of sea- 

 water, the lime, which gives the testacea their solid houses, 

 and the fish their bones, passing through the limestone rocks 

 would precipitate itself to the bottom in the shape of a bi- 

 carbonate, 



J. C, MANSEL. 



