5 1 8 THE JO URNAL OF GEOL O G Y. 



surface of the sea, and whose remains only enter into deposits 

 when they have escaped being used by other creatures, or being 

 dissolved in the ocean waters. 



Agassiz, writing of the physiology of deep sea life, 1 points out 

 that in marine, as in terrestrial, life the primary source of food 

 for animals is in plants. The lower types of marine life, it would 

 seem, must derive their sustenance from the water, as land plants 

 get theirs in part from the air, and the silica and lime thus ab- 

 sorbed is taken directly from solution ; but the creatures which 

 live on these forms, and the carnivorous animals that feed on 

 them, may get their lime and silica at second hand by digesting 

 and assimilating that which the lower types take from solution. 

 Thus the solids built from solution into organic tests may go 

 through numberless changes before they come to rest on the 

 bottom. 



Without pursuing the discussion of biological conditions favor- 

 able or unfavorable to deposition, and without entering upon the 

 question of coral formations, which are rarely of prominent inter- 

 est in stratified deposits, the writer wishes to consider only the 

 circumstances of limestone formation from organic remains, as 

 that from chemical precipitates has been considered. 



In discussing the solubility of shells in sea-water it has been 

 pointed out that the layer of organic matter which accumulates 

 at the sea bottom contains a solvent formed by the evolution of 

 carbonic acid in the process of decay. Through this layer all 

 substances must pass before they can become part of a lithified 

 stratum ; if they are plant tissue or flesh they will become more 

 or less oxidized ; if they are calcareous tests they will be more 

 or less completely dissolved, and, if there be any chemically pre- 

 cipitated lime, arriving on the sea bottom it, too, would be dis- 

 solved in this menstruum. The earlier forms of dredge which 

 scooped into the sea bottom, brought up a mass of ooze, formed 

 of fine particles, burying organic forms. The later forms of 

 dredge, arranged to skim the surface of the bottom, bring up 



J Op. cit., pp. 312-313. 



