358 On a Ternary Geological Classifieaiion. [July, 



by the Entomostraca seems to have been that of scavengers of 

 animal matter. 



Amongst the above generally minute organisms, the molluscs 

 lived and flourished, often contributing to the building of the hme- 

 stones ; and amongst the most important class of moUusca in this 

 respect were the Brachiopoda, placed by naturalists almost in the 

 lowest rank of this sub-kingdom. 



I shall close this branch of our subject by offering the following 

 synopsis of the chief limestone builders of successive geologic periods 

 in ascending order. 



Laurentiak. Fovfiminifera (?). Eozoon. 



Silurian and Devonian. Corals (chiefly of the orders Zoantharia tahulata, 

 and Z. rugosa of MM. IMilue-Edwards and 

 Haime), Crinoids, Brachiopods, and Entomostraca. 



Carboniferous ,. .. Corah {Zoantharia t'lbuluta, Z.7^i{gosa,Z.tabidosa), 

 Crinoids, Bryozoa, Brachiopods, and Entomos- 

 traca. 



Permian Corals (not abundant; Zoantharia tahulata, Z. 



rugosa'), Bryozoa, Conchifera, Entomostraca. 



Triassic Bryozoa, Echiuodcrms, Conchifera. 



Jurassic Corals (Zoantharia aporosa), Bryozoa, Echino- 



derms, Molluscs largely. 



Cretaceous Amorphozoa, Forarainifera, Corals (Zoantharia 



aporosa, Z. tahulata, Z. rugosa) Echinoid;B, 

 Bryozoa, Entomostraca, Brachiopods (Tere- 

 bratulaj). 



Tertiary Foraminifera (Nummulites), Corals (Zoantharia 



aporosa, Z. pjerforata, Z. tahulata), Echinoderms 

 (Echinoidce,, Asteroidce), and Ophiurida\ 



We have now to turn to the discussion of those strata of 

 mechanical origin which are associated with the hmestones. 



Sedimentary, or Inorganic, Strata. — In contrast with the 

 calcareous rocks, not only as regards origin but in mode of distri- 

 bution, we may include all those strata which are strictly mechanical, 

 such as conglomerates, sandstones, shales, clays with their several 

 varieties and combinations, whether metamorphic or otherwise. It 

 is not necessary to enter into a description of the maimer of forma- 

 tion of this class of strata, except to observe that from the more 

 rapid subsidence of the coarser particles which arc brought down 

 into the ocean and distributed over its bed — truncated sheets of 

 strata of this class will have a tendency to increase in thickness in 

 the direction of the source, or sources, of sediment. 



Now, the existence of sandy or muddy sediment in the waters of 

 the sea is well known to be detrimental to the growth, and even 

 vitality, of many of the delicate organisms which chiefly contribute 

 to the formation of limestones. On this point the evidence of 

 observers is conclusive as regards the present inhabitants of the 

 soa, and we may feel sure this adverse influence of muddy sediment 



