ON CORAL AND CORAL REEFS 137 



Halve, or quarter, this estimate if you will, in order 

 to He certain of erring upon the right side, and still 

 theftb remains a prodigious period during which the 

 ancestors of existing coral polypes have been undis- 

 turJSedly at work; and during which, therefore, the 

 climatal conditions over the coral area must have been 

 much what they are now. 



^.nd all this lapse of time has occurred within the 

 most recent period of the history of the earth. The 

 remains of reefs formed by coral polypes of different 

 k*ftids from those which exist now, enter largely into 

 the composition of the limestones of the Jurassic pe- 

 riod ; and still more widely different coral polypes have 

 contributed their quota to the vast thickness of the 

 carboniferous and Devonian strata. Then as regards 

 the latter group of rocks in America, the high authority 

 already quoted tells us : 



"The Upper Helderberg period is eminently the 

 coral reef period of the palaeozoic ages. Many of the 

 rocks abound in coral, and are as truly coral reefs as 

 the modern reefs of the Pacific. The corals are some- 

 times standing on the rocks in the position they had 

 when growing : others are lying in fragments, as they 

 were broken and heaped by the waves ; and others were 

 reduced to a compact limestone by the finer trituration 

 before consolidation into rock. This compact variety 

 is the most common kind among the coral reef rocks 

 of the present seas ; and it often contains but few dis- 

 tinct fossils, although formed in water that abounded 

 in life. At the fall of the Ohio, near Louisville, there is 

 a magnificent display of the old reef. Hemispherical 

 Favosites, five or six feet in diameter, lie there nearly 

 as perfect as when they were covered by their flower- 

 like polypes; and besides these, there are various 



