222 THE DAWN OP LIFE. 



the Challenger soundings, that in certain areas 

 of the South Pacific the ordinary foraminiferal ooze 

 is replaced by a peculiar red clay, which he attributes 

 to the action of water laden with carbonic acid, in 

 removing 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 decom- 

 position 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 glauconite left, or 

 even the latter may be decomposed, leaving silicious, 

 aluminous, and other deposits, which may be quite 

 destitute of any organic structures, or retain only 

 such remnants of them as have been accidentally or 

 by their more resisting character protected from de- 

 struction.* In this way it may be possible that many 

 silicious rocks of the Laurentian and Primordial ages, 

 which now show no trace of organization, 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. 



