FOSSIL REMAINS OF SPONGES. 565 



J147. There is some difficulty and uncertainty in regard to 

 the Fossil remains of Sponges ; but it is probable that these have 

 come down to us from a very remote period of the earth's 

 history, and it may be reasonably supposed that Sponges were 

 among the earliest inhabitants of the ocean. These remains are 

 found in two states. Sometimes the whole tissue has been per- 

 meated by siliceous or calcareous matter ; so that, on the mass 

 being broken, its internal structure is very evident. In other 

 instances we have only the casts, which have been formed by the 

 subsequent filling-up, with stony matter, of the cavities left by 

 their decay. There is reason to believe that, in the Sponges of 

 ancient date, the siliceous spicula must have predominated ; for 

 we find their fossil remains almost always silicified, even in 

 calcareous rocks. Thus, in the Chalk (in which they greatly 

 abound) all the remains of Sponges present the character of flints. 

 Some of these flints, when broken, exhibit very beautifully the 

 structure of the Sponge ; and others possess only its external 

 form. Now, many of the Chalk fossils are infiltrated with car- 

 bonate of lime, and not with flint ; and this even when associated 

 with Sponges. In the same flint-nodules which envelope 

 Sponges, the shells of the Echinus ( 1002) are found converted 

 into crystallized carbonate of lime, and dense shells of Mollusca 

 are scarcely at all changed. It is evident, then, that some 

 peculiar attraction for siliceous matter exists in the Sponges ; 

 and this is readily explained upon well-known chemical prin- 

 ciples. If in a saturated solution of two salts, a crystal of one 

 of those salts be placed, it will be increased by a crystalline 

 deposit of its own composition ; whilst the other salt will not 

 form any deposit around it. If two organic structures, there- 

 fore, one containing siliceous spicula, and the other calcareous 

 crystals, be exposed to a fluid holding both these substances in 

 solution, they will attract from it their own ingredient. 



