52 THEORY OF THE EARTlf. 



pidity, (heir stony tree-like fabrics are intertwined 

 and accumulated into the form of rocks and reefs, 

 and, rising even to the surface of the water, shut 

 up the entrance of harbours, and lay frightful snares 

 for navigators. The sea, throwing up sand and 

 mud on the tops of these rocky shelves, sometimes 

 raises them above its own proper level, and forms 

 islands of them, which are soon covered with a 

 rich vegetation. 



16. Of Incrustations. 



It is also possible that the animals inhabiting 

 shells may leave their stony coverings when they 

 die in some particular places ; and that these, ce- 

 mented together by slime of greater or less con- 

 sistence, or by some other means, may form exten- 

 sive banks of shells. But we have no evidence 

 that the sea has now the power of agglutinating 

 these shells by such a compact paste, or indurated 

 cement, as that found in marbles and calcarious 

 sand-stones, or even in the coarse limestone strata 

 in which shells are found enveloped. Still less do 

 we now find the sea making any depositions at all 

 of the more solid and silicious strata which have 

 preceded the formation of the strata containing 

 shells. In short, all these causes would not, 

 though combined, form a single stratum of any 

 kind, nor produce the smallest hillock, nor alter in 

 any perceptible degree the ordinary level of the 

 oeQan. 



