PORIFERA. 



159 



the original skeleton being now represented either by empty casts in 

 the rock or by pseudomorphs of lime, peroxide of iron, or iron- 

 pyrites. The general freedom of the body of the White Chalk 

 from silica can thus be readily explained on the ground that the 

 siliceous organisms which it, to begin with, contained, underwent a 

 more or less complete solu- 

 tion. The dissolved silica 

 thus obtained must, however, 

 have been from time to time 

 redeposited in a solid form, 

 thus giving rise to the nod- 

 ules of flint which are so 

 largely disseminated through 

 parts of the White Chalk. 

 On this view, the silica of the 

 Chalk-flints had an organic 

 origin, though deposited from 

 solution in sea -water. On 

 this view, it is also possible 

 to account for the fact that 

 the silica, in course of re- 

 deposition, should not only 

 have accumulated in a nod- 

 ular form round any foreign 



bodies which may have been lying on the sea-bottom at the time, 

 but should also have commonly penetrated into the internal cavities 

 of organic bodies, or should even have given rise to tabular masses 

 or to actual veins. 



In deposits of Tertiary age, finally, remains of Sponges have 

 hitherto been detected in much smaller numbers than in the pre- 

 ceding Mesozoic strata. The great group of the Pharetrones among 

 the Calcispongice. seems to have become extinct with the close of the 

 Cretaceous period, and the Hexactinellids and Lithistids are for the 

 most part but poorly represented. In the Miocene strata of Algeria, 

 however, Pomel has described an abundant fauna of Hexactinellid 

 and Lithistid Sponges. 



Fig. 51. — Section of chert from the Upper Green- 

 sand, showing numerous sponge-spicules embedded in 

 a chalcedonic matrix, enlarged forty diameters. (Af- 

 ter Binde.) 



