1869.] On a Ternary Geological Classification. 355 



Tabular View of Cycles in the PALiEozoic Age in Eastern America. 



I now proceed to explain the principles upon "wliich a three- 

 fold classification of strata with a calcareous central member is 

 based; in order to which a knowledge of the origin of marine 

 hmestones is essential. 



Marine Limestones. — And, first, this classification depends on 

 the distinctive character of marine limestones as compared with all 

 other stratified deposits. While conglomerates, sandstones, shales, 

 and clays are essentially mechanical in their mode of formation, 

 limestones (except under peculiar circumstances) are essentially 

 organic. This, Id deed, is the view of many of our most distinguished 

 naturalists, and amongst others of our great authority on chemical 

 geology, M. Bischof. This author states that the quantity of free 

 carbonic acid gas contained in the sea is five times as much as is 

 necessary to keep in a fluid state the quantity of carbonate of hme 

 to be found in it. From this he draws the conclusion that it is 

 impossible for any carbonate of lime to be precipitated in a solid 

 form at the bottom of the sea by chemical action alone, and as the 

 quantity is extremely small in the open sea, it is difiicult to conceive 

 that this can be efi'ected otherwise than by vital agencies.* 



This is a view, indeed, that can scarcely be questioned, and 

 which is supported by reference to the composition of the hmestones 

 themselves. If, indeed, we might have looked for illustrations of 

 this class of rocks as having been formed independently of organic 

 agencies, it would have been amongst the most ancient formations 

 of the globe. But what has been the case ? Far down below the 

 " Primordial zone" — in strata more ancient by at least two geologic 

 cycles than those lately supposed to contain the first traces of animal 

 life — we find a series of serpentinous limestones, of the real organic 

 origin of which, the microscope has not left us in doubt. The 

 researches of Sir William Logan and his colleagues of the Geological 

 Survey of Canada, followed by other naturalists, have demonstrated 

 that even the oldest known limestones on the surface of the globe 

 owe their origin to the Eozoon, an animal considered by Dr. Dawson 

 to be a Foramiuifer. 



* ' Chemical Geology.' 



