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On Coral Reefs and Tslands. 329 



r Art. XXXm.— 0?i Com? Reefs and Islands; by James D. 



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Dana. — Part Fourth. 



From the Report on Geology of the Exploring Expedition under Capt. Wilkes, VS.'S. 



Formation of Reefs, and Causes of their Features and 



Geographical Distribution. 



An inquiry into the causes and origin of the features presented 

 by coral reefs and islands, has led us to glance at the nature of 

 coral zoophytes, and at the effects of various agents upon their 

 de^relopment. The way has thus been prepared for considering 

 the bearing of these facts, and of other influencing causes, on 

 the growth of the coral plantation as a whole. While, therefore, 

 the preceding pages treat of zoophytes as individual species, the 

 following will relate to those results which proceed from their 

 accumulation, and the causes which have determined the features 

 and geographical distribution of reefs and islands. 



1. Formation of Reefs. 



^ery erroneous ideas prevail, respecting the appearance of a 

 bed or area of growing corals. The submerged reef is often 

 thought of as an extended mass of coral, alive uniformly over its 

 upper surface, and, by this living growth, gradually enlarging up- 

 ward: and such preconceived views, when ascertained to be er- 

 roneous by observation, have sometimes led to skepticism with 

 regard to the zoophytic origin of the reef-rock. Nothing is wider 

 from the truth : and this must have been inferred from the de- 

 scriptions already given. Another glance at the coral plantation 

 should be taken by the reader, before proceeding with the expla- 

 i^ations which follow. 



^ Coral plantation and coral field, are more appropriate appella- 

 tions than coral garden, and convey a juster impression of the 

 surface of a growing reef. Like a spot of wild land, covered in 

 some parts with varied shrubbery, in other parts bearing only oc- 

 casional tufts of vegetation over barren plains of sand, here a 

 clump of saplings, and there a carpet of variously colored flowers 



such is the coral plantation. Numerous kinds o( zoophytes 

 grow scattered over the surface, like vegetation upon the land: 

 there are large areas that bear nothing, and others that are thickly 

 overgrown. There is, however, no green sward to the landscape ; 

 sand and fragments fill up the bare intervals between the flower- 

 ing tufts: or where the zoophytes are crowded, there are deep 

 boles among the stony stems and folia. 



These observations will prepare the mind for some disappoint- 

 ment m a first view of coral reefs. Nature does not make green- 

 houses, but distributes widely her beauties, and leaves it for man 



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