WATER BISCUIT OF SQUAW ISLAND 



197 



the form of the biscuit as to make it clear that the calcareous de- 

 posit has been permeated with the organic matter. 



It is quite clear that the process of formation of these peculiar 

 bodies has been the following. The beach shale and debris have 

 become incrusted by a growth of algae^ and the latter, stealing 

 away for their requirements the excess of free carbon dioxid 

 in the water necessary to keep the carbonate of lime in solution, 

 have thus caused a precipitation of the lime salts. The process 

 has been continuous, as when a new precipitation formed a con- 

 centric continuous deposit of lime carbonate, the new surface 

 became coated with the algae and in consequence fresh precipi- 

 tation followed. The whole forms a most interesting instance 

 of the influence of plant growth on the formation of lime de- 

 posits. 



A group of Squaw island water biscuit. 



It is appropriate to note in this connection that European au- 

 thors have recorded the occurrence of similar spheric masses 

 of filamentous algae in various fresh and brackish lakes. 

 Some of these bodies from the lochs of the Hebrides have been de- 

 scribed and illustrated by Barclay, who designates them as algoid 

 lake haJls.^ 



1 G. W. W. Barclay, "Ou some algoid lake balls found in South Uist." Proceedings Koyal soc. 

 Edinburgh. 1886. 13: 845, pi. 30. 



