272 Dr Hibbert on the Limestone of Burdiehouse 



to its source, teems with fresh-water animals exclusively ; but, in 

 following it to its junction with the sea, marine products, even far 

 from its mouth, begin to appear. Yet the Ganges, notwithstand- 

 ing any partial ambiguity of its products, possesses not the less 

 a fresh-water character. 



Hitherto, however, I have not found the slightest traces of 

 marine mollusca or corallines in the limestone of Burdiehouse ; 

 and hence, I am not induced to consider it as any thing but a 

 pure lacustrine formation; and if any marine remains should in 

 future turn up, which I do not expect, they would be so small, 

 in comparison with the very abundant indications afforded of an 

 opposite state of the waters which had yielded this deposit, as to 

 merely indicate its original connection with an ancient sea, and 

 nothing more ; — which last supposition I have, from a different 

 description of evidence, never been once inclined to doubt. 



That seas should have been actually synchronous with a fresh- 

 water deposit, is indeed, as I have shewn, what might be natu- 

 rally expected. Few rivers or lakes subsist, which do not main- 

 tain such a communication. I am, therefore, prepared to expect, 

 that the remains of such large fish as are found in fresh-water 

 deposits, may also be discovered in marine deposits, analogous, in 

 fact, to what takes place at the present day in numerous parts of 

 the globe, where we find the fish which inhabit seas penetrating 

 many hundred miles up large rivers in quest of their prey, as, for 

 instance, towards the source of the Ganges. 



It is also very probable, when we consider the frequent oscil- 

 latory movements to which the crust of the earth was exposed 

 during the carboniferous epoch, that the fish which were then 

 created might possess an adaptation of animal structure calculat- 

 ed to enable them to sustain any occasional change of element 

 caused by terraqueous elevations or depressions ; for M. Agassiz 

 states, that, among the fish of the earlier formations of strata, he 

 has not observed those marked characteristics of structure, which, 

 in later times only* point out these animals as distinct inhabitants 

 of salt or of fresh water. 



