1869.] On a Ternary Geological Classification. 359 



extended far back through all geologic times. Taking the Polyps 

 as the chief hmestone builders of the present period, we find them 

 flourishing in mid-ocean, or in tracts removed from the influence of 

 turgid waters. Thus Mr. Darwin, when describing the coral reefs 

 and atolls of the Indian and Pacific Oceans, refers to them as being 

 at an immense distance from any continent, and where the water is 

 perfectly limpid.* Mr. Jukes describes the water surrounding the 

 barrier reef off" tlie coast of Australia as being quite clear ;t and 

 Dr. T. Wright, in his summary of the character and conditions of 

 development of modern coral reefs, says,:i: " where the bottom is 

 muddy, and the rivers pour fresh water in any great abundance 

 into the sea, there the reef-building Polyps are absent." 



The Foraminifera, a group of hmestone builders, distinct from 

 Polyps and Echinoderms, seem to have been most prolific in limpid 

 waters. In our present seas we find them largely associated with 

 the Polyps, as well as engaged in forming a deposit over the bed 

 of the mid-Atlantic, which if converted into land would yield 

 a limestone not dissimilar to chalk. § In geologic times we find 

 two very important and pure limestone formations — the chalk, and 

 nummulite limestone of the Eocene period, — for the most part com- 

 posed of the shells of these little animals. 



That the Brachiopods and the Crinoids especially of past times 

 flourished with greatest vigour in limpid seas, is perfectly clear 

 from the mode of their occurrence in the rocks themselves. Not 

 only are they most numerous as individuals in the beds of the less 

 earthy limestones, but we frequently notice the species (or their repre- 

 sentatives) becoming dwarfed in size, as well as fewer in numbers, 

 when we pass from limestones into adjacent clayey strata. In the 

 Carboniferous limestone of Derbyshire, where these animal remains 

 are found in such marvellous joroiusion, the rock itself contains scarcely 

 a band of shale through hundreds or even thousands of feet. 



If, then, there existed this detrimental influence, exerted by mud 

 or sand held in suspension, on the vital development of those marine 

 animals which contributed to the formation of limestones, it follows, 

 that during any special geologic period in which both classes of 

 strata (calcareous and sedimentary) were being foimed, the maxi- 

 mum development of each class must have been in directions 

 opposite to each other. If, for instance, the sediment was being 

 transported and deposited over the sea-bed by a current coming 

 from the north, the contemporaneously formed limestone would 

 grow with greatest rapidity, and attain its greatest proportions far 



* ' Naturalist's Voyage,' p. 468. 



t Voya-e of H.M. ship ' Fly.' 



X ' On Coral Eeefs, Past and Present,' Trans. Cottcswold Naturalists' Club, 

 1866. 



§ See Dr. G. C. Wallich's " Account of ^the Deep-Sea Soundings," ' Quart. 

 Journ. Science,' vol. i. 



