476 PRINCIPLES OF STRATIGRAPHY 



of the lime carbonate is brought about. Algae are most effective 

 in this respect, different algse accomplishing the separation in differ- 

 ent ways. According to Dr. Cohn (lo), who studied in detail 

 the algous limestones of the Carlsbad Sprudel, the lime is deposited 

 first in minute crystals in the slime between the vegetable, threads 

 and upon their surface. At first these crystals are separate, but 

 continued growth in numbers produces star-like clusters which by 

 enlargement grow into grains of calcareous sand. Further growth 

 results in the union of the grains into a solid mass of travertine. 



"The exact relation of the crystals and grains of carbonate of 

 lime varies in the dift'erent species of algae. In the Oscillar'kc of 

 Carlsbad, and allied species, the crystals form in the slimy intercel- 

 lular tissue ; in Haliiiicda, the carbonate of lime forms a sieve-like 

 cover about the tips of the algas filaments; and, in Acctularia it 

 occurs as a tube about the stalk of the plant. In the Charae the lime 

 is separated and deposited in the cells and cell walls of the back 

 alone, while in the Corallines [Corallina, Lithothamnion, etc.] it 

 is found only within the cells." (Weed-55 '.64 j.) In the last men- 

 tioned types, especially in Lithothamnion and allied forms, the 

 surface layer alone is living, the lime occupying the successive 

 strata of dead cells beneath. In this manner thick masses of lime- 

 stone are built up. Solution of the lime in acid sets free the dead 

 cell walls which will either remain behind as a tangled mass of fila- 

 ments, or float as pellicles on the surface of the solution. Rothpletz 

 found this to be the case in the green fission-algre secreting the lime 

 of the oolites as well as in the red algae, which form extensive lime- 

 stone deposits. 



Where calcium carbonate is absent, as in the Great Salt Lake, or 

 occurs only in small quantities, as in sea water, the lime-secreting 

 plants decompose the calcium chloride, phosphate or sulphate and 

 deposit it within their tissues as carbonate. The same is believed to 

 be true of many animals, which take the lime from the sea water 

 in the form of the sulphate, but deposit it in their shells and skele- 

 tons as carbonate. The reaction is brought about through the 

 formation of ammonium carbonate as a decomposition product, 

 which in turn, precipitates the calcium in the form of carbonate 

 from the solution within the dead cells. 



Separation of Siliceous Sinter by Plants. 



The silica is at first deposited in and upon the plants in the 

 form of a gelatinous substance, often quite brilliantly colored, in 



