156 SCIENCE PROGRESS 



mation of oolite grains is another instance : the collection of 

 the carbonate of lime is supposed to be brought about by the 

 thallus of an alga, which encrusts the central grain, depositing 

 calcium carbonate in concentric layers as it grows. Certainly 

 the encrusting red algae act in the same way but the living 

 organism has never been observed on the oolite grains, though 

 nodules of carbonate of lime in fresh-water lakes are usually 

 covered with blue-green algae, Glccocapsa, etc. The same process 

 is believed to give rise to the pisolites which separate in pea-like 

 granules as deposits from hot springs. The blue-green algae 

 can live in hot water but the actual organisms on the pisolites 

 have not been seen, though Cohn asserts their presence. 



On the other hand, the destruction of oolite grains and shells 

 generally is accomplished by boring algae, such as Hyella, 1 

 which send their microscopic filaments through and through 

 the hard calcite similarly to the mycelia of a fungus penetrating 

 rotten wood ; Lind, 2 in fact, found that fungi were actually 

 capable of sending their hyphae through marble. Boring 

 sponges like Cliona do the same. 3 



The separation of silica cannot be so easily explained. 

 There are countless plants and animals which absorb and 

 secrete silica, and the lower forms are usually closely allied to 

 the lime-secreting genera. Among the silicious plants there are 

 the hosts of the diatoms and among animals the radiolaria and 

 sponges. 



In the case of iron again, there are differences of opinion. 

 The bog-iron which forms at the bottom of lakes and under 

 the soil in marshy places, where it is known as moor-bed- 

 stone, ortstein or oude klip, is thought to be the result of 

 chemical deposition. Organic acids certainly dissolve the iron 

 and when the solution is oxygenated, carbon dioxide is given 

 off and the iron is deposited either as a carbonate or as a 

 hydrate. This is what happens in the laborator}' ; in Nature, 

 however, the precipitation goes on in the bottoms of lakes and 

 in soil which is not properly aerated. Ehrenberg attributed the 

 deposition to a diatom which he called Gallionella ferruginea. 4 



1 E. Bornet et Flahault, Note sur deux nouveaux genres d'algues perforantes, 

 Journ. Bot. vol. ii. 1888, p. 161 ; see also J. E. Duerden, Boring Alga?, Bull. 



Ann. Mus. Nat. Hist. New York, vol. xvi. 1902, p. 323. 



2 K. Lind, Jahr bitch, zviss. Bot. Bd. xxxii. 1898, p. 603. 



3 Topsent, Arch. Zool. Exper. (3), vol. viii. 1900, p. 226. 

 * C. G. Ehrenberg, Nukrogeologic, Leipzig, 1854. 



