278 Dr. J. D. Dana on the Origin of 



through the westward " sweep " of an eddy current to the 

 Gulf Stream. The subject, nearly twenty years afterwards, 

 was more thoroughly investigated by Mr. Alexander Agassiz, 

 and the same conclusion reached *. Mr. Agassiz made also 

 another important observation — that this current is an abundant 

 carrier of marine life for the feeding of the coral animals, and so 

 accelerates the coral-growth and accumulation in its direction. 

 Combining with these effects others hereafter considered, 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 irregu- 

 larities 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 



* On the Tortugas and Florida Keefs, by A. Agassiz, Trans. Amer. 

 Acad. xi. 1883. 



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

 in the U. 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 description 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 elevation 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. 



