1 6 SCIENCE PROGRESS. 



How far do other observations on the origin of oolitic 

 grains support the contention that Girvanella has in 

 many instances been instrumental in producing such rock 

 structure? On the shores of the Great Salt Lake of 

 North America there are accumulations of snow-white cal- 

 careous grains which the waves have washed up on the 

 beach ; these minute spherules correspond in the arrange- 

 ment ol the carbonate of lime, and in external form, to 

 typical oolitic grains. The nature of this modern oolitic 

 material has recently been made the subject of examination 

 by Rothpletz. 1 On dissolving the carbonate of lime he 

 finds that each grain shows a residue of algal cells, some 

 of which he compares to Gloeocapsa. Cells of this genus 

 and of Gloeotheca, surrounded bv the usual gelatinous 

 sheaths, occur as a coating to the calcareous spherules 

 whilst still in the waters of the lake. In all probability the 

 Salt Lake oolite is a product of algal life. Rothpletz's 

 researches in oolitic structure lead him to the conclusion 

 that at least the majority of marine calcareous oolites, which 

 exhibit a regular zonal and radial structure, must be looked 

 upon as the results of algal life. Another example of 

 oolitic structure closely connected with the growth of algae 

 may be quoted from an important work by Walther. This 

 author describes oolitic grains from the shores of the Red 

 Sea which have a central nucleus of quartz, felspar or gar- 

 net surrounded by carbonate of lime enclosing algal cells 

 and branched tubular structures.' 2 



Finally, Bleicher 3 has discovered organic matter in Juras- 

 sic oolitic grains from Lorraine, and in some cases this takes 

 the form of a tubular network of algal-like filaments. Here, 

 again, we have an organism, and presumably an alga, en- 

 tering into the composition and concerned in the formation 

 of oolitic grains. 



It appears from the researches of Rothpletz that in 

 some cases another form of alga closely allied to Girvan- 

 ella has played an important part in the production of 

 oolitic structure ; this is the genus Sphcerocodium, which he 



1 Rothpletz (2). 2 Walther (2), p. 482. 



3 Bleicher (1) and (2). 



