86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



in which the calcium bicarbonate is converted into the normal carbonate ' by 

 the oxygen liberated by the plants, and both carbon dioxide and oxygen set 

 free, the free oxygen possibly acting still farther to precipitate calcium 

 monocarbonate. 



He concludes that the alga Chara is the great agent for the con- 

 centration and precipitation of the calcium carbonate, and that the 

 Blue-green algae are also largely concerned in the formation of the 

 massive beds of lake tufa and the calcareous pebbles which show- 

 both radial and concentric structure. Dr. Davis describes the 

 pebbles as roughly ellipsoidal in shape, the radial lines shown in 

 the sections [Idem, p. 91] being formed by the growth of the 

 filaments while the concentric lines probably represent periods of 

 growth of the plants either seasonal or annual. Included within 

 this structure are great numbers of plants, besides the lime-secreting 

 Zonotrichia and considerable numbers of diatoms. These pebbles 

 have quite a wide distribution in the lakes of Michigan, Wisconsin 

 and elsewhere. 



Those interested should consult Dr. Davis's paper on the mineral 

 deposition of calcium carbonate through the agency of algae, also 

 Dr. John M. Clarke's paper on " Water Biscuit." ' Dr. Clarke 

 kindly sent me specimens of the " Water Biscuit " from Canandai- 

 gua Lake. A number of transverse sections were made of these, 

 some of which show a very distinct concentric structure (pi. 14). 

 Dr. Clarke in describing the origin of the " Water Biscuit " states 

 that : ' 



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 algse, and the latter, stealing away for their requirements the excess 

 of free carbon dioxide 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 concentric continu- 

 ous deposit of lime carbonate, the new surface became coated with the algae 

 and in consequence fresh precipitation followed. The whole forms a most 

 interesting instance of the influence of plant growth on the formation of lime 

 deposits. 



At my request Dr. Davis gave me the following notes on the cal- 

 careous deposits in Green and Round Lakes, situated two miles 

 (3.2 km.) southwest of Kirkville, Onondaga County, New York: 



These lakes are located in a deep valley which is apparently rock walled, 

 with the rocks covered from four to ten feet, apparently, with drift. The 

 walls of the valley are wooded and may be a hundred or more feet high. Green 



^ Which is only very slightly soluble, 100 parts to the million. 



*Bull. N. Y. State Mus., No. 39, Vol. 8, igoo, pi. 14, pp. 195-198, pis. 12-15. 



'Idem, p. 197. 



