96 History of 



duce its effects. This extinction, however, is not efl'ected with- 

 out very great resistance from the fire beneath. The water, 

 upon dashing into the cavern, is very probably at first ejected 

 back with great violence ; and thus some of those ajnazing 

 water-spouts are seen, which have so often astonished the mari- 

 ner, and excited curiosity. But of these in their place. 



Besides the production of those islands by the action of fire, 

 there are others, as was said, produced by rivers or seas carry- 

 ing mud, earth, and such like substances, along with their cur- 

 rents ; and at last depositing them in some paiticular place.* 

 At the mouths of most great rivers, there are to be seen banks, 

 thus formed by the sand and mud carried down by the stream, 

 which have rested at that place, where the force of the current 

 is diminished by its junction with the sea. These banks, by 

 slow degrees, increase at the bottom of the deep : the water in 

 those places is at first found by mariners to grow more shallow ; 

 the bank soon heaves up above the surface ; it is considered, for 

 a while, as a tract of useless and barren sand ; but the seeds of 

 some of the more hardy vegetables are driven thither by the 



* Islands of coral are also formed in tropical regions. Coral is the produce of 

 different species of vermes or worms, and it consists cliiefly of carbonate of lime. 

 Now it is difficult to conceive where these animals procure such prodigious 

 quantities of this substance. Sea- water indeed contains traces of sulphate of 

 lime, but no other calcareous salt, as far as is known. Hence it would appear, 

 that these creatures must either decompose sulphate of lime, though the qnan. 

 tity of that salt contained in sea- water seems inadequate to supply their wants, 

 or they must form carbonate of lime from the constituents of sea-water in 

 a way totally above our conception. Be that as it may, there is one con- 

 sequence of this copious formation of coral in the tropical regions of con.sider. 

 able importance to navigation. The winds and waves accumulate these 

 corals iu large banks, which, entangling the sand, gradually rise above the 

 surface of the vv'aves, and form islands. These, in process of time, probably 

 by the agency of birds, become covered with vegetation, and frequently load- 

 ed with timber. Mr Ellis, in his history of zoophytes, supposes that the 

 greater part of these numerous islands in the South Sea, have been formed by 

 coral, rising above the surface of the water. The bottom of these islands la 

 nothing else than a coral bank; the surface is a black soil, formed of 

 B mixture of sand and decayed vegetable matter; the whole island is 

 flat, long, and narrow ; and extends usually in its greatest length from 

 north to south, because almost all winds between the tropics blow either 

 from the east or the west. The sides of tliese islands frequently constitute ■} 

 perpendicular wall ; and the sea. at a little distance from them, is of an uufa 

 homable depth. 



