314 Mr Horner 07i the occurrence of the Megalichthys 



2. In connection with the absence of marine shells, the pro- 

 fusion of terrestrial plants. 



3. The presence of the remains of fishes that inhabited fresh 

 water, but which Dr Hibbert admits to be an ambiguous crite- 

 rion, (p. 271). 



4. The abundance of the shells of entomostraca, scattered 

 through the limestone. 



Let us now examine the weight of that evidence ; and first, as 

 to the absence of marine shells. 



In the immediate vicinity of Burdiehouse, there is a limestone 

 abounding in marine remains, which Dr Hibbert describes, and 

 which occurs in nearly conformable stratification with the other 

 coal-measures, and with the so-called lacustrine limestone. The 

 mere inspection of the diagram given by Dr Hibbert to shew 

 the relative position of the two beds of limestone would lead us 

 to conclude that they were deposited in the same waters, and 

 belong to one series ; and we know that nothing is more com- 

 mon than to find, in a series of strata, some beds of limestone 

 containing organic remains, and others in which not a trace of 

 an organized body can be discovered. Near Lul worth, in Dor- 

 setshire, where the Purbeck beds are largely developed, and 

 which abound in organic remains, there are compact varieties of 

 Purbeck stone, which are devoid of shells, and which attain a 

 thickness of from 60 to 100 feet.* Many of the beds of the 

 lias and oolite series of limestones, and which alternate with 

 shales and sandstones, are almost wholly made up of organic 

 remains, while others of the same series are wholly destitute of 

 them. The same thing has been observed in the carboniferous 

 limestone of Wales, of the north of France, and of Belgium. 

 Marine shells may not, as yet, have been discovered in the lime- 

 stone under consideration, but marine organic remains are abun- 

 dant in it, as I shall presently shew. But even beds contain- 

 ing exclusively fresh-water shells, in the opinion of geologists of 

 great authority, do not afford conclusive evidence of a lacustrine 

 deposit. In the memoir of Professor Buckland and M. De La 

 Beche, on the Geology of the Neighbourhood of Weymouth,-|- 

 the authors observe, " One of the most important points in the 



• GeoL Trans. 2d ser. vol. iv. p. 12. t Ibid. 



