GEOLOGICAL CONCLUSIONS. 353 



great width. The substratum, however, is, in general, contin- 

 uous coral-rock; and if these more elevated parts were re- 

 moved by any process, after an elevation, they would leave a 

 nearly level area of coral limestone often as extensive as the 

 whole reef-grounds. This is at once seen from the map of the 

 islands of the Gilbert Group (p. 165), or that of the Feejees. 

 In an island like Dean's, one of the Paumotus, these reef- 

 grounds are 1,000 square miles in extent. 



But the most extensive reef-grounds of the oceans are after 

 all of small breadth compared with many of the ancient lime- 

 stones of the continents ; and the reef-rocks also are peculiar 

 in their very abrupt limits, the margins sometimes descending 

 at a steep angle a thousand feet or more. These differences 

 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 generally formed over the broad surface 

 of a continent as it lay slightly submerged. 



The Abrolhos reefs of the Brazilian coast, described on page 

 140, illustrate one of the methods by which the coral banks ex- 

 tend 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 anv 

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

 avoidable 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 



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