178 J. D. Dana — Origin of Coral Reefs and Islands. 



conclusion reached.* Mr. Agassiz made also another im- 

 portant observation — that this current is an abundant carrier of 

 marine life for the feeding of the coral animals, and so accele- 

 rates the coral growth and accumulation in its direction. 

 Combining with these effects others considered beyond, Mr. 

 Agassiz expresses, like Mr. Murray and Mr. Semper, the 

 further conclusion, that all kinds of reefs, atoll, fringing and 

 barrier, may be made without aid from subsidence. 



a. The facts, presented by Lieutenant Hunt, and more fully 

 by Mr. Agassiz with regard to the effects of the eddy current 

 of the Gulf Stream, show that coral reefs may be elongated, 

 and also that inner channels may be made, by the drifting of 

 coral sands. But the action with coral sands is essentially the 

 same as with other sands ; and illustrations of this drifting 

 process occur along the whole eastern coast of North America 

 from Florida to Long Island. We there learn that drift-made 

 beaches run in long lines between broad channels or sounds 

 and the ocean ; that they have nearly the uniform direction of 

 the drift of the waters,, with some irregularities introduced by 

 the forms of the coast and the outflow of the inner waters which 

 are tidal and fluvial and have much strength during ebb tide. 

 The easy consolidation of coral sands puts in a peculiar feature, 

 but not one that affects the direction of drift accumulation. 



b. The great barrier reef off eastern Australia, a thousand 

 miles long, has some correspondence in position to the sand- 

 reefs off eastern North America. But it is full of irregulari- 

 ties of direction and of interruptions, and follows in no part 

 an even line. In the southern half, it extends out 150 miles 

 from the coast and includes a large atoll-formed reef; in the 

 northern half, the barrier while varying much in course is 

 hardly over 30 miles from the land. There is very little in its 

 form to suggest similarity of origin to the drift-made barriers 

 of sand. 



c. In the Pacific Ocean, the trends of many of the coral 

 island groups, and of the single islands, do not correspond with 

 the direction of the oceanic currents, or with any eddy cur- 

 rents except such as are local and are determined by them- 

 selves. 



*On the Tortugas and Florida Reefs, by A. Agassiz, Trans. Amer. Acad., xi, 

 1883. 



Professor Louis Agassiz's account of the Florida reefs was published in the 

 TJ. S. Coast Survey Reports of 1851 and 1866, and reproduced in vol. vii of the 

 Memoirs of the Museum of Comparative Zoology. It gives an excellent descrip- 

 tion of the Florida reefs, and of the action of boring animals and other injurious 

 agents on corals, and reaches the conclusion that the reef has been raised to its 

 present level and thickness by wave and current action, without the aid of eleva- 

 tion or subsidence. The argument is based on such observations as could be 

 made over the surface of the reefs and the adjoining sea-bottom, and bears on 

 the question of the necessity of subsidence and not on the fact of subsidence. 



