6 ANCESTRAL MAN. 



But the formation of Flint, which is pure silex and not a salt of 

 silica, cannot be explained quite so easily. Because, in the first 

 place, silica itself is absolutely insoluble in water ; and in the 

 next place silicification is not a constant but an intermittent 

 phenomenon. 



Flint occurs in irregular but for the most part independent 

 nodules that vary in size from particles of microscopic minuteness 

 to masses three feet in diameter. A careful examination shows 

 that they consist, in most cases, of silicified sponges, corals, 

 sea-urchins, or molluscs ; while in other cases the silicious material 

 is aggregated round a nucleus of animal or vegetable structure. 



Chert, however, consists of a silicious agglutination, either of 

 particles of sand as in cretaceous beds, or of minute silicified 

 organisms, as in calcareous rocks. 



Now, as on the one hand, flint nodules, no matter what their 

 basis, are found arranged in definite layers, which correspond 

 with the bedding of the chalk but which are separated from one 

 another by intervals of entirely cretaceous material ; so on the 

 other hand, chert, however formed, and whether in broad or 

 narrow bands, alternates with beds of limestone. 



In this striking way the problem emerges, that whereas in 

 ocean- waters the materials both for silicification and for calcification 

 are always present, these processes go on not coincidently, but 

 in an alternating and highly irregular manner and so assuredly 

 depend on some " reciprocating " oscillatory causal agency that 

 favours this process as it swings in one direction, and that process 

 as it swings in the other direction. 



The materials for silicification exist abundantly in the sea ; but 

 some way must be shown by which silica, insoluble in water, can 

 be removed from its salts and deposited in organic tissues. Now, 

 solutions of silicates, in the presence of some acids, and notably in 

 the presence of hydrochloric and carbonic acids, throw down a 

 gelatinous hydrate of silica which dissolves in excess of the acid ; 

 and by dialysis through organic membranes a pure soluble silicic 

 acid is left, which undergoes a spontaneous decomposition into a 

 colloidal silica. 



At the bottom of the sea, then, we have the constant presence 

 of salts of silica, of animals and, consequently, of carbonic acid, 

 of hydrochloric acid, and of dialysing membranes. Yet silici- 

 fication is not constant but intermittent. 



The materials for calcification also exist abundantly in the ocean : 

 lime, carbonic acid, and decaying organic structures ; and yet 

 calcification, too, is irregularly intermittent. 



During the third cruise of H.M.S. Porcupine, a number of 

 specimens of salt-water were procured for analysis ; and in every 



