264 Dr Hibbert on the Limestone of Burdiehouse, 



ceived its excessive proportion of carbonic acid, was purified by the development of 

 the numerous plants, particularly of gigantic species, which were created during the 

 carboniferous epoch, and for the support of which an extraordinary quantity of car- 

 bon must have been required. 



All these views are unexceptionable so far as they go. There can be little doubt 

 that the development of the abundant vegetation of coal-fields must have rendered 

 solid much carbon which had existed in the atmosphere ; — but* that this development 

 was eventually, that is, during a still later formation of rocks, the cause of ren- 

 dering the atmosphere fit for sustaining the respiration of reptiles possessing lungs, 

 is another question ; as we have no data whatever for ascertaining if such an amount 

 of carbonic acid had subsisted, as to really render the atmosphere incompatible with 

 reptilian vitality. On the other hand, the researches explained in the present memoir 

 inform us, that an animal closely resembling the Lepidosteus, which M. Agassiz has 

 proved to actually possess lungs, subsisted during a period when it was supposed 

 that no animal possessing lungs could have possibly breathed. 



SECTION XI SUMMARY OF THE EVIDENCE RELATIVE TO THE FRESH-WATER 



ORIGIN OF THE LIMESTONE OF BURDIEHOUSE. 



The fresh-water limestone of Burdiehouse has at length been 

 described, in which it has been shewn, that this bed is nothing 

 more than an unit amidst many other fresh-water deposits, vary- 

 ing in their mineralogical character, and alternately deposited in 

 the great Lowland basin of Scotland. 



In concluding this memoir, I shall bring the fresh-water evi- 

 dence to as severe a test as is compatible with geological reason- 

 ing, which must too frequently, in its very nature, be little more 

 than analogical and circumstantial. In insisting upon a more 

 rigid species of evidence, too many geological systems which we 

 have carefully reared, would disappear " like the baseless fabric 

 of a vision." 



The fresh-water evidence of the Burdiehouse limestone has 

 hinged upon various points. 



The first has been the absence of all mollusca and conchifera 

 of acknowledged marine origin. 



In recent times, the conchifera and mollusca of marine origin, 



