GEOLOGICAL SIGNIFICANCE OF SPONGES 379 



the Cretaceous period". Again there is no evidence to support this. Neither in the 

 complexity of the skeleton, the size of the spicules, the size of individual sponges, nor 

 in the diversity of species, even when allowance be made for the fact that many species 

 living in Cretaceous times are as yet unrepresented in our collections, can the Cretaceous 

 fauna be said to have reached a degree of development greater than that represented by 

 the sponges of the Antarctic to-day, or, for that matter, any other quarter of the globe. 



And, finally, there is the question of the segregation of silica in the sea, in relation to 

 the formation of chert and flint. Some geologists find it difficult to believe that the vast 

 quantities of silica contributed annually to the sea by rivers (320,000,000 metric tons) 

 can all be accounted for by the silica deposited as animal skeletons,^ even allowing for a 

 small percentage known to be deposited in a colloidal state in the normal course of 

 sedimentation. Judging by the sponge fauna of the European or American coasts, there 

 is doubtless some ground for such doubt, but if the Antarctic be considered too the 

 case assumes a difl^erent aspect. Here, in a vast expanse of ocean, with the erosive forces 

 calculated to contribute silica to the sea reduced to a minimum, the growth of siliceous 

 sponges is greater than in any other part of the world : and it can only be supposed that 

 the supply of silica in this area is drawn from other quarters. Even more important, 

 however, is the suggestion that, when it is considered how many siliceous sponges were 

 collected by the Discovery expeditions at a relatively few pin-points of the Antarctic 

 expanse, these 3 20,000,000 metric tons are probably barely sufficient to meet the demands 

 of all silica-secreting organisms, sponges, radiolaria, etc. In fact, the assumption is 

 more probable that, in addition, a fair proportion of the silica from the skeletons of dead 

 sponges, and other organisms, may enter once again into circulation in the sea. 



There is another point, from which no definite conclusions can be drawn, but the 

 mention of which may be of interest. Among all the Antarctic collections represented 

 in the British Museum, numerous examples may be found of accumulations of spicules 

 into irregularly rounded masses, or balls (see Plate LVI I , figs. 14-16). These are formed of 

 the larger spicules of siliceous sponges, matted and felted together in compact masses up 

 to 20 cm. in diameter : and their occurrence suggests that, indirectly, sponges may be the 

 agents for a considerable amount of the segregation of silica necessary for the formation 

 of chert and flint, in a manner hitherto unsuspected. There can be little doubt that these 

 " spicule balls " are formed in much the same way as the balls of Posidonia fibres, by the 

 action of waves, and it is probable that consideration of their presence in large numbers 

 on the sea-bottom, taken in conjunction with the suggested continuous extrusion of 

 sponge spicules during life (cf. Burton, Proc. Zool. Soc. London, 193 1, p. 524) may be 

 of importance in the study of cherts and their origin. 



1 See Tarr, Univ. Missouri Studies, 1926, Vol. i, No. 2, p. 2. 



