vol. 62.] halimeda as a reee-eorming organism. 711 



Discussion. 



The Chairman (Mr. R. S. Herries) called attention to the great- 

 interest of the communication, as throwing new light upon^ the 

 question of the formation of limestones. 



Prof. Watts drew attention to the Nullipore-reefs described 

 originally by Sir John Murray. The speaker referred to the 

 ' Eichthofen' reefs in the dolomites, and pointed out that the coral- 

 origin of these had been discredited, partly because they contained 

 few corals. The present paper showed that corals might be a very 

 minor contribution to actual ' coral-reefs.' 



Prof. Jobcnston-Lavis said that he wished to draw attention to 

 some papers by Prof. Johannes Walther, in which that investigator 

 expressed the opinion that the calcareous algse played an important 

 part in the origin of dolomites. 



Prof. Judd, replying on behalf of the Authors, stated that the 

 great interest of the paper depended on the circumstance that a 

 calcareous green alga was now shown to play an important part 

 in limestone-building. The freshwater Cliara and the purple sea- 

 weeds of the type of Lithoihamnion had long been known to 

 form the basis of limestones. In answer to a question asked 

 by Dr. Robert Bell, he pointed out that, when all structure was 

 destroyed in a limestone by crystallization, its coral-reef origin 

 might be inferred from the smallness of the insoluble residue in this 

 class of rocks. In reply to Prof. Watts, he recalled the important 

 results arrived at by Prof. Skeats in his study of the dolomite-rocks 

 of the Tyrol. 



Q.J.G.S. No. 248. 3 c 



