ALGM AS ROCK-BUILDING ORGANISMS. 21 



tion of phycologists, as well as increased recognition on the 

 part of geologists. 



A recent discovery of Lithothamnion in a cutting at 

 the mouth of the River Liffey deserves a passing notice. 

 In deepening the extremity of a well in 1871-72 several 

 shells which had been dug up in the process of excavation 

 were found to be grown over with some calcareous organisms. 

 These encrusting growths were determined to be species of 

 Lithothamnion, very like L. polymorpha and L. fascicidatinn, 

 both of which are still living in the Bay of Dublin, but no 

 longer exist in the Liffey. 1 



ALG.'E AND HOT-SPRING DEPOSITS. 



In an exhaustive monograph on this subject Weed ex- 

 presses his belief that " the geological work of plant life has 

 not been generally recognised, partly because it is less con- 

 spicuous, and partly because the absence of organic remains 

 in many deposits formed in this way has prevented a recog- 

 nition of the true origin of the rocks". 2 The able account 

 by this geologist of the Yellowstone Park hot springs 

 should have the effect of raising the importance of the 

 lower forms of plant life as geological agents. 



Records of alga^ in the waters of hot springs have been 

 supplied by various observers, and the power of such plants 

 to withstand high temperatures has been long known. 

 Weed has done good service in calling attention to the con- 

 sequences of such algal life in thermal waters. The exact 

 nature of the vegetation in the Yellowstone Park springs is 

 often difficult to determine, the plants being frequently en- 

 crusted and hidden by such substances as sulphur, sul- 

 phate of calcium, oxide of iron, etc. In the district of the 

 Mammoth hot springs, extensive masses of travertine or 

 calcareous tufa have been gradually built up. The springs 

 themselves vary in temperature from 8o° to 165 F., and all 

 have afforded numerous examples of actively living plants. 

 The growths vary considerably in character and colour 

 according to the temperature of the water ; in rapidly flow- 



1 O'Reilly. z Weed, p. 619. 



