304 CORALS AND CORAL ISLANDS. 



between the new and the old arise in part from the fact that 

 the coral reefs of the present era are made about small oceanic 

 lands, or along the edges of the continents, while the limestones 

 of ancient time were gradually formed over the broad surface 

 of a continent as it lay slightly submerged. 



The Abrolhos reefs of the Brazilian coast, described on page 

 I IT, illustrate one of the methods by which the coral banks 

 extend and finally coalesce into beds of wide extent ; but these 

 are small compared with the great limestones of early time, 

 and owe their slight approximation to them as regards extent 

 to the wide range of shallow waters there afforded. These 

 Abrolhos reefs differ from most limestone beds also in being 

 formed largely of the corals in the position of growth. 



The tendency of modern reefs to grow up to the surface in 

 narrow banks, separated by channels, appears to be unlike any- 

 thing we discover in the old rocks ; and it seems to be an 

 unavoidable result of growth in the sea, where the waves pile 

 up barriers, and the currents make, and keep open, channels. 

 The case of the Australian and Feejee reefs are good examples. 

 It is possible that such barriers may often have existed in 

 ancient time, and have disappeared through subsequent denu- 

 dation of the surface. But may not the difference between the 

 great even layers of the continental formations and those of 

 a coral island have proceeded from the difference in the depth 

 of the seas ? Over the great shallow continental seas where 

 the limestones were in progress, the waves may have generally 

 been feeble, and therefore there may have been a less tendency 

 to form narrow barriers and deep intervening channels. 



The marsh condition of a drying-up lagoon with its forming 

 limestones has been compared above with that under which 

 ancient unfossiliferous limestones were made. The narrt)w 

 limits of the former make the comparison unsatisfactory ; for, 

 in the coral island, coarsely fossiliferous beds are all the while 

 forming about the exterior of the island, but a few miles at 

 the most from the lagoon-marsh ; while the ancient limestones 

 retain their unfossiliferous character often through many thou- 

 sands of square miles. Still, the above-mentioned difference 



