ALG^ AS ROCK-BUILDING ORGANISMS. 23 



calcareous springs, here too a gradual transition may be 

 traced from the living gelatinous growths to hard and com- 

 pact sinter. Calot/irix gypsopliila, Leptothrix and other 

 genera are recorded from these sinter-forming springs ; 

 such plants are spoken of as algae, but probably they would 

 be more fitly described as bacteria. Some forms of the 

 siliceous sinter appear to have been mainly formed by a 

 species of moss, Hypnum aduncwm var. grasilesiens Br. 

 and Sch. 



The causes which have operated in the formation of 

 sinter are summarised as follows : Relief of pressure, cool- 

 ing, chemical reaction and evaporation. 



This subject of sinter-building by low forms of plants 

 was discussed at some length in an important communica- 

 tion from Prof. Cohn * in 1862, in which he demonstrated 

 the importance of such forms in the deposition of the Carls- 

 bad " Sprudelstein ". 



When we take account of the occurrence of algae and 

 bacteria in the hot springs of New Zealand, Iceland, India, 

 the Azores, Malay Archipelago, Japan, America and other 

 places, it must be admitted that Weed had good grounds 

 for asserting that the geological work of the lower forms 

 of plant life has not been generally appreciated. 



SIPHONED VERTICILLAT^E. 



Did space permit some account might be given of the 

 numerous calcareous algae in the Triassic limestones of the 

 Southern Alps. Such genera as Dactylopora, Haplopor'- 

 e//a, 2 Gyroporella, Uteria and many others were originally 

 described as foraminifera by Carpenter, Gumbel and other 

 writers. Gumbel included in the family Dactyloyidece 3 a 

 rich variety of forms possessing calcareous skeletons, and 

 his papers contain valuable information as to the struc- 



1 Cohn. 



2 It should be noted that Rothpletz has recently pointed out that 

 Haploporella fasciculata Gumbel is not an alga, but consists of echinoid 

 spines (Neues Jahrb., vol. i., 1891, p. 285). 



3 Gumbel, p. 253. 



