184. THE OCEAN. 



northern coasts of Sicily, where the mean temperature of the waters 

 rises to 64-4 Fahr., the stones and pebbles of the shore are, in many- 

 places, agglutinated by calcareous cement.* In the same way the 

 fragments of rocks, which the torrents of Arabia Petraja bring every 

 winter from the top of the mountains to the shores of the Red Sea, 

 are, in the space of a few weeks, converted into a stratum of solid 

 conglomerate. Every year a new layer of stone is added to the old 

 ones, and in future centuries we shall be able perhaps to estimate the 

 age of the formation by the number of its beds, one above the other, 

 in the same way as we recognize the age of a tree by the number of 

 its annual rings of wood.f 



We must explore the shores of the Antilles, or other tropical seas, 

 to observe this phenomenon of the formation of rocks in all its gran- 

 deur. There the waves, heated to 89'6 Fahr. by the rays of a 

 vertical sun, deposit limestone in sufficient quantity notably to in- 

 crease the extent of the shore. The tufa of Guadeloupe, in which 

 the famous human skeleton exhibited at the Britisli Museum was 

 found, belongs to this recent formation. It grows, so to say, under 

 the very eyes of the observer, and gradually covers with a rocky 

 crust all those oi)jects which the sea rejects, and which the brooks 

 bring down from the interior. In many parts of Terra Firma these 

 quarries of marine stone are actively worked for building towns on 

 the coast, and all the excavations made in these banks of limestone 

 are soon filled up by new materials. The quarry grows under the 

 labourers who are occupied in detaching the blocks ; hence the name 

 of 31agonne-bon-dicu, which the natives have given to those rocks 

 which seem to be renewed of themselves. 



On the shores of Ascension Island Mr. Darwin found some of 

 these conglomerates cemented by marine limestone, whose specific 

 weight was 2*63, that is to say, hardly less than that of Carrara 

 raarble. These beds of compact stone, deposited by the sea, contain 

 a certain quantity of sulphate of lime, as well as the animal sub- 

 jstances which are evidently the colouring principle of the whole mass. 

 Sometimes the translucid stucco covering the rocks has the polish, 

 hardness, and hue of nacre ; moreover, as chemical analysis proves, 

 this kind of enamel and the envelope of living molluscs are composed 

 of the same substances equally modified by the presence of organic 

 matter. Mr. Darwin has seen some of these calcareous deposits, 



* De Quatrefages, Souvenirs d'un naturaliste. 

 t Marsh, Man and Nature, p. 455. 



