76 Calcareous Pebbles formed by Alga. 



water of a high temperature ; hence in the cold polar and deep sea waters 

 of our day no massive carbonate of lime shells or other structures are 

 secreted by organisms.' Although plainly to be otherwise accounted for, 

 it is interesting to observe that the coralline plants of the sea are much 

 more massively developed in the warmer seas than in the colder ones. In 

 fresh waters it would be interesting to know how far the incrustation of 

 CJiaracece, for example, is affected by temperature. Certainly, in the 

 particular case under discussion, the massive occurrence of carbonate of 

 lime suggests a hot-spring Alga rather than one living at the temperature 

 of a lake in Michigan. The incrustation of carbonate of lime so often 

 found on the surface of plants is, no doubt, often due to the evaporation of 

 water holding the salt in solution, which has been excreted by the plant ; 

 but, as Vines (Lectures on the Physiology of Plants, p. 22) suggests, ' in the 

 cases of submerged plants it is possible that the calcium carbonate may 

 be deposited on the surface in consequence of the absorption of carbonic 

 acid from the water by the plant.' Since a relatively high temperature 

 would encourage this operation of dissociation, it is exceedingly probable 

 that we have in such a process an explanation of this distribution of 

 carbonate of lime deposits according to temperature. Gomont (loc. cil.} 

 says of OscillariecB that, when they live in waters strongly charged with 

 lime, the precipitation of carbonate of lime is caused by the decom- 

 position of carbonic acid, and that this occurs only where the Algai do not 

 find carbonic acid in sufficient quantity to meet the needs of vegetation. 

 When it is abundant, as in certain mineral waters, the salt is precipitated 

 only in very small quantities. 



In the Annals of Botany (vol. v., p. 225), Mr. Thiselton-Dyer refers to 

 the occurrence of round calcareous pebbles in profusion at the bottom of 

 Lough Belvedere, near Mullingar. He collected and examined these 

 pebbles, and was much struck with their geological possibilities. ' An 

 accumulation of them might easily form a rock, and the interpretation of its 

 structure would not be easy.' The pebbles in this case (fig. 6) are not 

 nearly so large as those from Michigan, and Mr. Thiselton-Dyer describes 

 them as 'of all sizes up to that of a filbert.' He has most kindly placed at 

 my service a number of these pebbles, and in his letter calls my attention to 

 the resemblance of ' the material to pisolite [= Pea grit of the Inferior 

 Oolite, not the pisolite limestone of Eocene Age] though, of course, the 

 structure is very different.' I have seen a specimen of this rock from the 

 Cotteswolds, and it so exactly resembles the appearance that would be 

 presented by a heap of these pebbles that I am in no way surprised at Mr. 

 Thisclton-Dyer's insisting on the geological interest of these bodies. Those 

 pebbles I have cut open are solid, though the substance is less dense at the 



