THE SURFACE OF THE GLOBE. 37 



their bottom shoals of mud, which would in time choke them up, if 

 constant care was not taken to cleanse them ; the sea also leaves in 

 harbours, creeks, and all parts where its waters are most calm, mud 

 and sediment. Currents are formed amongst these deposites, or throw 

 upon them the sand which they collect from the sea ; and thus are 

 shoals and shallows made. 



Staliacttes. 



Certain waters, after depositing the calcareous substances, by means 

 of the superabundant carbonic acid with which they are impregnated, 

 become crystallized when the acid has evaporated, and form stalactites 

 and other concretions. There are mingled crystallized layers in soft 

 water, sufficiently extensive to be compared with some of those left by 

 the ancient sea. Every one knows the famous Travertine quarries in 

 the vicinity of Rome, and the rocks of this stone which the river Teve- 

 rona accumulates and produces, perpetually varying in form. Its 

 twofold action may be thus accounted for : the accumulated deposites 

 of the sea may become hardened by stalactites ; when, perhaps, springs 

 replete with calcareous matter, or containing some other substance in 

 solution, fall into the places where these masses are formed, and then 

 become a combination formed by the union of the marine deposites 

 with the fresh water. Such are the banks of the Guadeloupe, which 

 contain marine and terrestrial shells and human skeletons. Such, also, 

 is the quarry near Messina, described by Saussure, where the sand- 

 stone is produced by the sands cast up by the sea, and which there 

 consolidate. 



Litophytes. 



In the torrid zone, where litophytes are numerous in species and 

 propagate rapidly, their stony columns are formed into rocks, reefs, &c. ; 

 and, reaching to the level of the waves, block up the entrance of the 

 ports, and are the destructive foes of navigation. The sea casting 

 sand and slime on the top of these rocks, frequently raises their sur- 

 face above the proper level, and thus generates islands which soon 

 exult in rich vegetation.* 



Incrustation. 



It is possible, also, that in certain places shell-fish leave their testa- 

 ceous coverings, which, amalgamated with slime more or less con- 

 creted, or with some other cements, form extensive deposites, or a 

 kind of shelly reef ; but we have no evidence that the sea at present 

 can incrust these shells with a paste as solid as marble, sand- stone, 



* See Forster's ' Observations on the South Sea.' 



