in the Neighbourhood of Edinburgh. 27 '1 



As for the remains of Cestraciontes, (and perhaps of the Me- 

 galichthys,) which appear in more than one description of car- 

 boniferous limestones, they point to estuaries, no less than to 

 fresh-water lakes, as having been in primeval times frequented 

 by large animals in quest of their prey. 



The foregoing researches shew, — 



1st, That it is by the presence or absence of acknowledged 

 pelagic mollusca, corallines, &c. that indications are afforded of 

 the great difference between fresh-water and marine deposits. 



9.dly, That if, along with marine mollusca or corallines, we 

 find the plants of coal-fields in a quantity comparatively small, an 

 estuarian limestone may be inferred. And, 



Zdly, That if marine mollusca or corallines should be entirely 

 absent in a limestone, and if plants should be abundantly found in 

 it, an indication would be afforded of a calcareous deposit which 

 took place amidst the fresh-water rivers or lakes of primeval 

 marshes ; which indication would be still more favoured, if we 

 should find, in addition, recognised genera of fresh-water shells, 

 the entomostraca of stagnant marshes, or the fish incidental to 

 coal-fields. 



It is however admitted, that the presence of fish is an ambi- 

 guous criterion. The smaller fish of lakes, or rivers, are known in 

 recent times to venture into estuaries, while larger fish enter fresh- 

 water rivers, or lakes, in pursuit of their prey. 



From these conclusions it is evident, that while a broad line of 

 demarcation subsists between marine and fresh- water limestones, 

 it is by no means impossible that limestones of an estuarian and 

 fluviatile formation may, in some cases, be more difficult to dis- 

 tinguish, and particularly, when it is kept in view, that, from a 

 remote period of the globe, communications between rivers and 

 seas must have subsisted, as at the present time Thus, during 

 the actual state of our globe, the Ganges, as we approach close 



