in the Neighbourhood of Edinburgh. 265 
as well as of fresh waters, are distinctly recognised. We never wit- 
ness seas inhabited by successive races of unios, or fresh-water 
rivers filled with a succession of coralline substances. If proof’ 
have been brought forward that an interchange of element among 
animals dwelling in seas, or in fresh-water lakes or rivers, is pos- 
sible, it is certain that such an existence could not long be main- 
tained with immunity. Vitality would be rather tolerated than 
fostered, and this would be shown in the gradual diminution and 
eventual obliteration of the races of animals thus subjected to an 
interchange of element,—which fact at least indicates, that mol- 
lusea and conchifera,.in their distribution, bear reference to difter- 
ent mediums of saline and fresh water for their prolonged support. 
This principle I have analogically applied to the limestones 
of the carboniferous epoch. If we find that marine mollusca and 
conchifera are present in most limestones in the vicinity of Edin- 
burgh, yet are absent in the limestone of Burdiehouse, to what 
other conclusion does this absence point, but that a condition of 
waters had prevailed, during the time of this deposit, unfavour- 
able to the vitality of marine animals? And hence the supposi- 
tion, that this condition was a difference of element, similar to 
that which still prevails, namely, fresh water, hostile to the pro- 
longed existence of pelagic conchifera and mollusca. 
A second point upon which the evidence of the fresh-water 
origin of the Burdiehouse limestone has hinged, is circumstantial 
rather than analogical. 
The circumstantial evidence is to the following effect :—That 
the calcareous deposit of Burdiehouse must have taken place in 
a depression, or basin, perfectly surrounded with a dense vegeta- 
tion, which has been washed into inland waters. But this cir- 
cumstance would of itself prove little, as we may easily suppose 
that an estuary, or arm of the sea, might have stretched through a 
tract where adense vegetation has prevailed. But when, in con- 
nection with a perfect absence of all acknowledged marine remains 
VOL. XII. PART I. | 
