Forbes — Chemistry of the Primeval Earth. 441 



or cold, they would have, from the moment of their deposition, 

 possessed a decided crystalline structure, visible when examined by 

 the microscope ; as in the case of stalactites, stalagmites, travertines, 

 etc. ; this, however, is not the case. 



Sorby's microscopical researches prove satisfactorily that all 

 limestones, from the most ancient up to the most recent, are solely 

 formed of the debris of organisms,' and that they do not possess any 

 crystalline structure whatsoever, unless when altered by subsequent 

 infiltration, or other metamorphic action. 



Dr. Hunt next proceeds to explain that the magnesian limestones, 

 dolomites, and gypseous beds owe their origin to chemical '•' reactions 

 hitherto unsuspected," and that his experimental researches have 

 proved them to have been formed at a period when the surface of 

 the earth was covered by a dense atmosphere of carbonic acid, and 

 that this " theory is confirmed by climate, by vegetation, and by the 

 singular series of reactions which hitherto have been a perplexity to 

 chemical geologists." ^ 



The microscopical and chemical investigations of Sorby have, 

 howcA^er, eliminated the most conclusive evidence against the cor- 

 rectness of this theory of precipitation, and shown the magnesian 

 limestones and dolomites alluded to by Dr. Hunt (whether of the 

 Devonian, Carboniferous, or Permian period) to be mere mechanical 

 aggregates, or true limestones of ordinary character subsequently 

 altered by infiltration of magnesian matter. 



This result had been long before arrived at by geologists, as the 

 study of these rocks in the field showed that such magnesian lime- 

 stones were frequently only portions of the ordinary limestone beds 

 peculiar to the formation itself, altered at places, apparently by some 

 then unexplainable chemical action. This was found to be the case 

 even with limestones pertaining to the Devonian and Carboniferous 

 formations, in which period it has long been advanced that an 

 atmosphere rich in carbonic acid did exist. 



As all geologists know that the grand development of magnesian 

 limestones, dolomites, and gypseous beds really took place in an 

 epoch when numerous air-breathing animals, both vertebrates and 

 invertebrates, lived upon the face of the globe, it will surprise them 

 to think that Dr. Hunt can imagine these aniinals living in an atmo- 

 sphere of carbonic acid. 



The next point in this lecture to which attention is directed is a 

 very important one in its general bearings, although it is to be feared 



1 Even the chalk is entirely so composed, notwithstanding that its external appear- 

 ance is so like that of a precipitated carhonate of lime. 



2 Dr. Hunt seems to be quite unaware that in the Brit. Assoc. Eeport, 1856, p. 77, 

 Sorby has fully explained these reactions, that Harkness (Brit. Assoc. Report, 1857, p. 

 68) applied similar experimental investigations of Regnault, to explaining the dolorai- 

 tization of the Carboniferous limestones near Cork; and, lastly, that the results of his 

 researches on the artificial atmosphere of carbonic acid, which he has thought worthy 

 of bringing before the French Academy (Compt. Ilend., 1867, p- 815), so far from 

 being new, have for the last twenty-two years at least, if not much longer, been 

 employed on a large scale in the manufacture of magnesian compounds in both 

 England and Ireland. 



