150 THREE CRUISES OF THE " BLAKE." 



acid, in regions where no rapid oxidation takes place, either 

 from currents, or waves, or from other atmospheric influences. 



An immense amount of silica must find its way into the sea, 

 and be immediately dissolved by the excess of carbonic acid 

 found near the bottom, while only a portion of the calcareous 

 mud can be taken up in solution. Hence, this silica is at once 

 placed under the most favorable conditions for resorption by 

 organisms living upon the layer of protojjlasmic substance 

 which covers the bottom of the ocean, into which the silica has 

 been received. As Wallich and others have most distinctly 

 proved, this protoplasmic layer, where it exists, is the product 

 of the organic life, and not its source. 



Wallich gives some most excellent reasons for supposing that 

 the silex nodules found in the chalk owe their origin to sponges. 

 This is additional evidence in favor of the opinion, which is 

 gradually gaining ground, that the deep-sea calcareous mud is 

 but a chalk deposited in the present epoch, and in every way 

 to be compared with the chalks of the cretaceous period ; and 

 further still, that this dejDOsition of chalk has been going on 

 uninterruptedly from cretaceous times, and that we may be said, 

 as far as that special deposit is concerned, to be living in the 

 period of the chalk, for no lithological distinction of any value 

 has been established between the chalk projjer and the calca- 

 reous mud of the Atlantic. 



The Atlantic ooze is mainly made up of the calcareous casts 

 of globigerinse, and their sarcode must have contributed in no 

 small degree to the protoplasmic mass of the bottom. It must 

 have been an important addition to the silica supplied by 

 sponges in the seas where siliceous foraminiBera are abundant.^ 

 As an additional source of supply of silica we have the exten- 

 sive deposits of diatoms, which fall to the bottom after death. 

 A similar condition of things probably existed during creta- 

 ceous times, and the chalk flints were derived from the same 

 groups of animals which now supply the silica and the globi- 

 gerina ooze of the Atlantic. 



^ Sir Heiiry De la Beche states, that ter, but if allowed to remain too long in 

 siliceous particles are generally dissemi- suspension the silex will become aggre- 

 nated in water if mixed with clayey mat- gated into small lumps. 



