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 rilled with a succession of coralline substances. If proofs 

 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- 

 lusca and conchifera, in their distribution, bear reference to differ- 

 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 a dense vegetation has prevailed. But when, in con- 

 nection with a perfect absence of all acknowledged marine remains 



VOL. XIII. part i. l 1 



