264 RELICS OF PRIMEVAL LIFE 



the action of water laden with carbonic acid, in re- 

 moving all the lime, and leaving this red mud as a 

 sort of ash, composed of silica, alumina, and iron 

 oxide. Now this is in all probability a product of 

 the decomposition and oxidation of the glauconitic 

 matter contained in the ooze. Thus we learn that 

 when areas on which calcareous deposits have been 

 accumulated by Protozoa are invaded by cold arctic 

 or antarctic waters charged with carbonic acid, the 

 carbonate of lime may be removed, and the glauco- 

 nite left, or even the latter may be decomposed, 

 leaving silicious, aluminous, and other deposits, 

 which may be quite destitute of any organic struc- 

 tures, or retain only such remnants of them as have 

 been accidentally or by their more resisting character 

 protected from destruction.^ In this way it may be 



^ The "red chalk" of Antrim, and that of Speeton, contain 

 arenaceous Foraminifera and silicious casts of their shells, 

 apparently different from typical glauconite, and the extremely 

 fine ferruginous and argillaceous sediment of these chalks may 

 well be decomposed glauconitic matter like that of the South 

 Pacific. I have found these beds, the hard limestones of the 

 French Neocomian, and the altered greensands of the Alps, 

 very instructive for comparison with the Laurentian lime- 

 stones ; and they well deserve study by all interested in such 

 subjects. 



