lxxiv PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. lxxix,„ 



now "being formed, that oolitic structure generally is evidence of 

 the presence of living plants. Some oolitic grains and the circular- 

 or oval cake-like masses, known in America as ' water-biscuits,' 

 found in freshwater lakes, owe their origin to algal agency. On 

 the other hand, oolitic grains have been formed artificially b} r the 

 action of sodium carbonate on calcium sulphate. Moreover, the 

 association of plant-cells with grains or pebbles of carbonate of" 

 lime may be secondary; the algae may have invaded the lime- 

 deposits, and need not necessarily have had any share in their pre- 

 cipitation ; their presence may be accidental, not causal. The 

 balance of opinion Avould seem to be against the assumption that 

 the formation of concentric shells of lime necessarily implies the 

 presence of alga? or other plants. Further reference is made to 

 the association of organic structures and oolitic grains in a later 

 section of this Address. There are, however, many recent alga? 

 which have calcareous coral-like bodies ; the walls of the living 

 cells become impregnated with carbonate of lime, and the soft- 

 bodied alga is converted into calcareous branches or larger 

 encrusting coral-like masses based on a framework of plant-tissue. 

 These are the calcareous alga? of recent seas which occur in arctic 

 regions, though mainly in tropical and sub-tropical waters, and in- 

 marine strata of many geological periods. The abundance of the 

 genus Litlwtliamnion in the North Polar sea, where the tem- 

 perature rarely rises above 0° C, is a fact to be reckoned with when 

 use is made of calcareous alga? as a measure of climate. In recent 

 years substantial additions to our knowledge of the structure and 

 distribution of Pala?ozoic representatives of these reef-forming 

 plants have heen made by Prof. E. J. Garwood, ably assisted by 

 Miss E. Goodyear. It is noteworthy that the majority, at least, 

 of the specimens described by authors as calcareous alga? from 

 Archaean rocks are not in the strict sense alga? at all, but are 

 bodies varying in form and size, built up of concentric layers or- 

 exhibiting an irregular sponge-like texture. They show no cellular 

 structure like that of calcareous alga?, and the only evidence of" 

 any connexion with alga? is that, in a very few of them, groups or 

 chains of cells have heen discovered on the removal of the cal- 

 careous matrix by the action of an acid. Some limestones in 

 Ontario, well down in the Archaean System, reaching a thickness of' 

 500 to 700 feet, have been described as ' almost an aggregate of 

 fossils'. These supposed fossils, which reveal no cellular structure,, 

 consist of calcareous bodies. 1 to 15 cm. in diameter, characterized 



