426 E. S. MOORE 
arrangement that they seem to indicate the replacement of organic 
cells. 
That these larger concretions are the result of the action of 
algae, which cause precipitation of calcium carbonate owing to the 
chemical changes produced in the water by them, there seems to be 
little doubt. They are very similar to the concretions described 
from the Algonkian rocks of Montana by Walcott, and the cal- 
careous concretions described by Roddy? as now forming in the 
streams of Lancaster County, Pennsylvania (Fig. 13). Numerous 
Fic. 13.—Recent algal concretion collected by J. Roddy in Conestoga Creek. 
Lancaster County, Pennsylvania (2.5 inches in diameter). 
other occurrences of similar deposits have been described by other 
writers. The two types, large and small, bear certain resemblances 
to Walcott’s Newlandia concentrica and Collenia? frequens, but 
they do not seem to be identical with them, and new generic and 
specific names should be applied. 
As stated above, no definite organic cell structure has so far 
been recognized in these large concretions, but the replacement 
™C. D. Walcott, op. cit. 
2H. J. Roddy, ‘‘Concretions in Streams Formed by the Agency of Blue-Green 
Algae and Related Plants,” Proc. Amer. Phil. Soc., LIV, No. 218 (August, 1915), 
pp. 246-58. 
