part 2] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxiii 



coaly substance, occur parallel to the planes of bedding. It is 

 suggested by their discoverer that these bodies are imperfectly- 

 preserved plants : as records of plant-life they are valueless. 



The occurrence of iron-ore in association with Archaean rocks 

 has been adduced as evidence of the existence of plants. It is 

 well known that certain bacteria and other micro-organisms absorb 

 iron from the water in which they live, and ferric hydroxide is 

 deposited. Much has been written on the relation of bacteria to 

 ferruginous deposits by H. Molisch, D. Ellis, E. C. Harder, and, 

 more recently, E. S. Moore ; but it would seem impossible to 

 determine by any direct evidence whether the formation of iron-ore 

 in the past should be ascribed to the action of micro-organisms. 

 Phosphatic nodules from Torridonian rocks in Scotland have re- 

 vealed, on microscopic examination, an appearance recalling irregular 

 groups of small-celled tissue superficially (at least) similar to that of 

 simple plants, although the resemblance may be purely accidental. 

 Limestones are by no means rare in the Archaean S} r stem, and for 

 the greater part they have not yielded any satisfactory records of 

 plant-life. Dr. Walcott in 1915 recorded the discovery in sections 

 of an Upper Archaean limestone in Montana of cells and chains of 

 cells, which he considered to be bacteria of the Micrococcus type. 

 "Such they may be ; but, except in very favourable circumstances, it 

 is extremely difficult to draw the line between ciystals, or embryo- 

 crystals, and bacterial cells. It has been demonstrated that 

 unfossiliferous calcareous beds may have been formed by the agency 

 of plants. Deposits of chalky mud are now accumulating west of 

 the Bahamas and in the neighbourhood of the Keys of Florida as the 

 result of the precipitation of carbonate of lime by denitrifying 

 bacteria. The examination of calcareous reefs in the Mediter- 

 ranean, which unquestionably owe their origin to masses of lime- 

 secreting algae, has demonstrated that portions of the rock have 

 lost all traces of their plant-origin. While some of the Archaean 

 limestones may have been produced through the agency of plants, 

 the occurrence of calcareous rocks cannot reasonably be regarded 

 as evidence of organic origin. 



The abstraction by algae of carbon-dioxide gas from water 

 causes the precipitation of carbonate of lime ; the bicarbonate in 

 solution is thrown down as the insoluble carbonate. The frequent 

 association of minute tubes and other presumably plant-structures 

 with oolitic grains in rocks of different ages has led some authors 

 to infer, from the analogy of a similar association in grains 



