NATURAL HISTORY OF MARL 497 



important contributors to marl beds. It may be said in passing 

 that the limy incrustations which are \ found upon twigs, 

 branches, shells, and other objects in lakes and streams, and 

 called generally "calcareous tufas," are of similar origin and are 

 formed by nearly related, if not by the same, plants that form 

 the pebbles. 



Studies have been begun by the writer to solve, if possible, 

 some of the questions which have arisen in connection with the 

 statements embodied in this paper, but enough has already been 

 done to show that these forms of fresh-water algae are important 

 lime-precipitating agents now, and to suggest the possibility that 

 in all likelihood they have been more active in former geological 

 times, and that, as has been suggested again and again by botan- 

 ists, the formation of certain structureless limestones, and tufa 

 deposits may have been due to their work. 



Charles A. Davis. 

 Alma College, 



September I, 1900. 



