IX. OBSERVATIONS ON SOME CALCARE- 

 OUS PEBBLES. 



CHALMER POWELL. 



The first account of pebbles formed by algae in the United 

 States is given by Dr. George Murray (i). The pebbles ex- 

 amined by him were found in eight feet of water on the sandy 

 bottom of a Michigan pond separated from Lake Michigan by 

 a sand-bar. The specimens varied in size from one to three 

 and a half inches in diameter, were hollow and showed a strati- 

 fied or concentrically zoned structure. Upon decalcification they 

 were found to be composed of a densely interwoven mass of fila- 

 ments. The predominating kind was a species of Schizothrix, 

 S . fasciculata Gomont. There were also filaments of Stigonema 

 and Dichothrix and a large number and variety of diatoms. 



Mr. Thiselton-Dyer (2) refers to the occurrence of pebbles on 

 the bottom of Lough Belvedere, near Mullingar, which were 

 "of all sizes up to that of a filbert." The bulk of the algal 

 mass consisted of a Rivularia. 



The first mention of calcareous pebbles in American literature 

 was made by Professor Conway MacMillan (3), who states that 

 although he had " not yet found any of these algal pebbles 

 in lakes of Minnesota, it is probable that they occur." This 

 prediction was realized in June, 1901. 



In describing pebbles found along the shore of Littlefield 

 lake, Michigan, Mr. Charles A. Davis (4) states that they are 

 "the result of the development and growth of an alga, Zono- 

 trichia, or a nearly related species. The vegetable origin of 

 these pebbles would not be suspected until one recently taken 

 from the water is broken open, when it is found to show a 

 radial structure of bluish-green lines." In a preceding article 

 the same author (5) describes a blue-green alga concerned in 

 the formation of marl, which had been determined to be a 

 species of Zonotrichia or some closely related genus. "The 

 plant grows in relatively long filaments formed by cells grow- 



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