264 Dr Hizzert on the Limestone of Burdiehouse, 
ceived its excessive proportion of carbonic acid, was puritied 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. Acassiz 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, 
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