250 MR. A. W. WATERS ON FOSSIL LITHOTHAMNIA. 



up from the water and convert into carbonate, may have 

 upon the question of the formation of dolomitic rocks. 



The Lithothamnium takes very various forms — in some 

 formations abounding as small concretionary -looking 

 lumps, in others in branched form. Some recent ones at 

 the British Museum are tubular, others filiform ; some 

 anastomose; very frequently others grow quite straight. 

 One Melobesiacea in the British Museum is composed of 

 fine filiform tubes, about six or eight together, these 

 bundles then frequently anastomosing. The fossil ones 

 are very frequently much altered in colour. One rock on 

 the Lake of Garda might almost be described as a black 

 rock with white spots, an infiltration having taken place 

 from the outside, turning all but the central portion a 

 dark colour, so that each piece, when broken through, 

 shows the centre white and the rest dark. I bring this 

 before your notice to show the great care that is re- 

 quired when judging from lithological characteristics, as 

 no one would, at first sight, and often not without 

 microscopical examination, consider such a pure white 

 rock, and one almost black, to be composed of the same 

 material. 



From the mode of its growth, it will easily be under- 

 stood that solid masses can be formed of such stony sea- 

 weeds, since they grow incrusting stones, corals, and 

 other Lithothamnia, in this way filling up all the inter- 

 stices. In some instances, in rock- sections, I have found 

 the chambers of Foraminifera thus filled up; probably 

 they were partially broken before the commencement of 

 the vegetable growth. 



As I before said, the difficulties are at present very 

 great in studying this organism, on account of the want of 

 material and observation ; but it is very important every 

 geologist should look carefully for it, since it should be a 

 very material help in regard to the climate, and the con- 



