ALG.E AS ROCK-BUILDING ORGANISMS. 19 



Numberless nodules of Lithothamnion ramulosum and L. ra- 

 cemus are brought up in the dredgings from this calcareous 

 bank ; also foraminifera, gasteropods, diatoms and other 

 organisms. By the action of percolating water the Litho- 

 thamnion structure is gradually obliterated, and the calca- 

 reous mass becomes a structureless limestone. 



Walther applies his knowledge of this recent algal de- 

 posit to the examination of a Tertiary " Nulliporenkalk ,: 

 near Syracuse. In many parts of this formation there 

 occur well-preserved specimens of Lithothamnion, but in 

 others a gradual obliteration is observed of all plant struc- 

 ture until the rock becomes entirely structureless. A simi- 

 lar instance of structureless limestone is described from the 

 Lias of Todten Gebirges ; the strata consist of coral rock, 

 detrital calcareous deposits, and, associated with these, 

 masses of limestone in which microscopic examination fails 

 to detect either vegetable or animal structure. These 

 structureless beds are considered to have been Lithotham- 

 nion banks from which percolating water has removed all 

 trace of algal cells. It is susfPfested that the infiltrating 

 water was supplied by the Lithothamnion thallus with the 

 necessary amount of carbonic acid, and was thus enabled to 

 remove all direct evidence of the existence of calcareous 

 algae. In connection with this solvent power of the water 

 Walther asks the question: "What becomes of the plant 

 cellulose in the process of fossilisation ? " An instructive 

 comparison is made between the chemical composition of 

 compact Lithothamnion masses from the Secca di Penta 

 palumno and the Tertiary Lithothamnion limestones in 

 the neighbourhood of Syracuse ; in the former the CaCo 3 

 reaches 86%, and the organic substance 5% ; in the latter the 

 CaCo 3 reaches 98%, and organic substance 0*2 8%- The 

 organic substance of the algae became chemically altered in 

 the Syracuse beds, and in the course of such changes car- 

 bonic acid was evolved ; this was readily taken up in solu- 

 tion by percolating water, which was thus supplied with the 

 means of obliterating all traces of Lithothamnion structure. 



Thus it is shown that in masses of calcareous algal re- 

 mains there is an "endogenous source" of carbonic acid 



