THE GEOLOGIC FUNCTIONS OF LIFE. 655 



dant in the surface-waters of the temperate and tropical seas, and as 

 important in contributing to the calcareous deposits of the sea-bottoms. 

 The affinities of these bodies are in doubt, but they are regarded by 

 Murray as probably pelagic algae. 



The stone worts (Charace?e), an aberrant group of algse inhabiting 

 fresh and brackish water, secrete notable quantities of calcium carbonate 

 in and around their tissues, and the accumulation of these gives rise to 

 marl or limestone. It has recently been urged that our so-called shell- 

 marls are mainly due to Char^/ the molluscan shells being incidental 

 rather than essential constituents. 



In very ancient and also in some of the later strata, there are lime- 

 stones that do not carry any visible fossils, and their origin is, therefore, 

 debatable. There are also not a few limestones that are made up of a 

 fine-grained base through which are scattered molluscan shells, corals, 

 etc., in a fine state of preservation. The condition of these fossils bears 

 rather adversely on the view that shells, etc., have been powdered in 

 sufficient numbers and to a sufficient degree to form the compact base. 

 In all these cases the usual explanations leave something to be desired. 

 It is worth considering whether low forms of plants may not be among 

 the undemonstrated agents in forming these apparently unfossiliferous 

 limestones or parts of limestones. The calcium carbonate deposited 

 by the algse is in minute and delicate form, and is usually crystalline 

 while yet in the living tissues. It is, therefore, easily subject to com- 

 minution and to such further crystallization as would obscure the 

 minute features that constitute the evidences of algal origin. 



The more complex and conspicuous algae, the seaweeds, have left 

 impressions of their stems and fronds on the marine beds of most of the 

 periods, but they are usually obscure. Seaweeds are perhaps the source 

 of the vegetal matter in certain carbonaceous shales and limestones. 

 As seaweeds extract bromine and iodine and certain metallic ingredients 

 from the sea-water, some of the iodine and bromine springs issuing from 

 ancient marine deposits, and certain ores, may owe their origin to ancient 

 seaweeds. 



Diatoms, minute plants of the Thallophyte group, secrete a delicate 



framework of silica which becomes a contribution to the silicious deposits. 



Diatoms have sometimes contributed the material for very considerable 



beds, such as those of the ooze-bogs now forming in the marshes of the 



1 C. A. Davis. Jour, ©f Geol, Vol. IX, 1901, p. 491. 



