500 FOSSIL INVEKTEBRATED ANIMALS. 



doubtful, we say, that the astraese, the caryophylli, produce 

 with the same rapidity as the escharse. Moreover, it is indu- 

 bitable that these stationary animals, not being able to live, 

 either at depths where the action of the solar light and heat is 

 not exerted, which we know positively to be the case with the 

 true coral, nor sufficiently near the surface of the sea, to expe- 

 rience the violent movements with which it is often agitated, 

 and, still less, out of this surface, even supposing all the other 

 conditions to be most favourable, nothing greater can result 

 than some strata a few fathoms in thickness. It is, then, most 

 palpably evident, that the islands, archipelagos, and reefs, 

 with which the Indian and Southern oceans are so thickly sown, 

 cannot be entirely madreporic, as has been for a long time 

 supposed, but merely portions of a soil analogous to that of 

 the nearest continents, and for the most part entirely volcanic, 

 which have been encrusted with madreporic depositions of 

 greater or less thickness. Such is the opinion of MM. Quoy 

 and Gaimard, naturalists on the expedition of Capt. Freycinet, 

 who, having visited the same points as Peron, and, among 

 others, Timor and the Isle of France, have endeavoured to 

 demonstrate, in a memoir on the growth of lithophyte polypi, 

 considered geologically, that all that has been asserted or be- 

 lieved to the present day, relative to the immense labours 

 which the saxigenous polypi are capable of executing, is in- 

 exact, excessively exaggerated, and most frequently erroneous. 

 It may be, however, that these observers have underrated the 

 influence of the saxigenous polypi in the composition of the 

 islands and reefs of the seas of those torrid climates. They 

 begin by premising that the encrusting polypi, such as the 

 astrsea, the caryophilli, &c., which are evidently those whose 

 limits of growth are the most extended, or the least confined, 

 are also those which live at the shallowest depths ; for they 

 say that they have never met with them below a few fathoms. 

 Nevertheless, by their own avoAval, the branching polypi can 

 live at much greater depth ; for these authors mention an 



