140 CORAL REEFS. 



Alluvions formed by the sea are very similar ; they contain marine debris 

 of every kind, sometimes alone and sometimes mingled with fluviatile and 

 /erreslrial debris, brought into it by rivers. Debris of human industry, an- 

 chors, boats, &c., are frequent, and even man's remains exist ; not only in 

 cemeteries of villages that have been overwhelmed by sands, but also among 

 the debris cast up by the sea, as at Guadaloupe, where human bones are 

 found in a sand consolidated by a calca'reous tu'fa, and mingled with debris 

 of human art. In deltas formed partly of fresh water and partly by the sea 

 we find alternate layers, the one filled with marine debris, and the others by 

 those of fresh water; but, under other circumstances, all these remains are 

 found indiscriminately mingled. 



Argilla'ceous, marly, .or calca'reous deposits, in lakes, contain the remains 

 of fluviatile and terrestrial mollusks, similar to those now existing in the 

 eame regions. Remains of fishes and mammals are also occasionally found. 

 There is no doubt deposits formed in the sea also contain remains of 

 the numerous animals that daily perish. We learn from soundings that the 

 bottom of the sea, in many places, is covered by shells, broken or entire, 

 fragments of madrepore, echinidse, &c., sometimes mingled with sand, 

 sometimes by themselves, constituting considerable banks in progress of 

 formation and consolidation. 



39. Coral reefs. Formations of stony polypa'ria, agglomerated 

 with each other, often of great extent, are thus named ; in inter- 

 tropical regions they constitute a great number of islands, on a level 

 with the sea, or submarine banks, the mass of which rises more 

 and more. It is scarcely twenty years since it was supposed that 

 the little animals which form these deposits, by a calcareous exu- 

 dation, had the faculty of living at great depths in the ocean ; it 

 was thought they began their dwelling, and gradually augmented 

 the mass, until it formed immense mountains, the summits of 

 which constituted the reefs, and that they gave origin to most of 

 the large islands formed in those regions. These microscopic 

 creatures, it was said, tended thus to fill up the ocean, and were 

 preparing prodigious changes on the surface of the globe. But all 

 this exaggeration has disappeared, the observations of MM. Q,uoi 

 and Gaimard having shown, that the species which contribute most 

 to the formation of reefs, such as caryophy'llix (Jig. 220), mean- 

 dri'nse (Jig. 221), and particularly the as'tresc (Jig. 222), which 

 sqrnetimes cover immense spaces, and various madrepores (fig. 

 228), cannot exist except at moderate depths, and ten or twelve 

 yards below the surface no trace of them is to be found. It is, 

 then, on pre-existing rocks, already elevated under water, often 

 very steep on the sides, as soundings show, that these animals 

 begin to build ; and from this they afterwards accumulate their 

 solid product to the level of the sea, where their last generations 

 perish. They cannot, then, fill up the ocean ; but the incrusta- 

 tions they form are not the less important, since they are sometimes 

 ten or twelve yards thick, extending over immense spaces, and 

 these are found in a great many places in all seas comprehended 



39. In what parts of the world do we find coral reefs ? How are they 

 formed ? At what, depths do polypa'ria live ? 



