VERTEBEATES. 15 



two causes: either a superabundant development of ichthyic 

 life during the accumulation of the sedimentary material that 

 forms these thin beds of limestone, or else (and more probably) 

 from the sudden introduction of heated waters or noxious gases, 

 that caused a general and overwhelming destruction of the finny 

 tribes of the Carboniferous ocean at these different periods. 

 Such destructive agents may have been introduced, either by 

 the action of submarine volcanoes or of thermal springs, and it 

 is perhaps quite impossible, at the present time, to determine 

 to which of these causes, if either, we are indebted for the 

 phenomena we are now considering. 



We know, from the recorded observations of the last century, 

 that a submarine outburst of volcanic fire carries destruction to 

 the finny tribes of the ocean as far as its influence extends, 

 and we may readily conceive that the prevalence of thermal 

 springs beneath the ocean's level, the waters of which were 

 charged with mephitic gases, would produce a similar result. 

 If we accept the commonly received opinion that the siliceous 

 material which, in the form of chert and hornstone, enters so 

 largely into the composition of the Burlington and Keokuk 

 limestones, was introduced by thermal springs, we must con- 

 clude they were exceedingly numerous during the Sub-carbon- 

 iferous period, inasmuch as we find these materials so abundant 

 in some beds of the Sub-carboniferous limestone series as to 

 constitute from one-fourth to one-half of the entire mass. Now, 

 if we suppose these thermal waters to have been charged with 

 any poisonous ingredient, we have an agent sufficiently potent 

 to produce a very general destruction among the marine ani- 

 mals that were subjected to its influences. 



With a profound conviction that the suggestions thus thrown 

 out are not altogether a satisfactory answer to the query pro- 

 pounded, we have nevertheless felt impressed to offer them as 

 the most probable solution presented by the facts before us, and 

 with the hope that they may lead to further investigation and 

 a more satisfactory answer to this interesting inquiry. 



