626 DYNAMICAL GEOLOGY. 



mon feature of coasts like that of eastern North America. In the cases of the barriers 

 about the islands of the Pacific, however, there is no question on this point. Such bar- 

 riers do not form about so small islands. Moreover, the great distances of the reefs 

 from the shores, in many cases, and the existence of islands representing all the steps 

 between that with a fringing reef and the true atoll, leave no room for doubt. The re- 

 moteness of the Australian barrier from the continent, and the great depth of water in 

 the wide channel, show that this reef is unquestionable proof of a subsidence, — though 

 it is not easy to determine the amount. Along the shores of continents, the question 

 whether a barrier coral reef is evidence of subsidence or not must be decided by the 

 facts connected with each special case. 



Recapitulation. — The following are some of the points, connected 

 with the formation of limestone strata, illustrated by coral reefs : — 



1. The narrow geographical limits of coral-reef rocks at the present 

 time, owing to the existing zones of oceanic temperature. 



2. The narrow limit in depth of the reef-making corals, — this 

 depth not exceeding one hundred feet. 



3. The promiscuous growth of the corals over the reef-grounds. 



4. The perfect compactness and freedom from fossils of a large 

 proportion of the coral rock, although made within a few hundred feet 

 of living corals and shells ; the oolitic structure of part of this com- 

 pact kind ; while a variety made of broken corals cemented together 

 is common on the seaward side of a reef, and another, made of stand- 

 ing corals with the interstices filled, forms where there is shelter from 

 the ocean's waves. 



5. The aid of the waves of the ocean necessary, for making a solid 

 limestone out of corals or ordinary marine shells. 



6. The great extent and thickness of single reefs. 



7. The action of tidal currents and those arising from the piling in 

 of the waves during stormy weather, in keeping open channels and 

 harbors, and determining the distribution of the coral detritus. 



8. The close proximity, along shores bordered by barrier-reefs, of 

 deposits of coral material and deposits of river or ordinary shore 

 detritus. 



9. An exceedingly slow subsidence, in progress during the growth 

 of the corals, the cause of the change of a fringing reef into a barrier, 

 and later into an atoll. 



10. The necessity of this subsidence, for giving great thickness to 

 such limestones. 



The making of limestones from shells or crinoidal remains is similar 

 to that from corals, the waves wearing them or part of them to sand 

 or mud, and then consolidation taking place. The rate of formation of 

 limestones from shells is slower than that of Coral or Crinoidal lime- 

 stones, since Mollusks produce in their calcareous secretions much less 

 carbonate of lime in proportion to their bulk. 



