54 DR. H. B. GUPPY. 
blocks and reef débris, and the lagoon channel is formed in 
the same way as the lagoon of an atoll. This is but a more 
precise and more scientific statement of the natural growth 
of reefs. Whilst Professor Semper relies more on the 
agencies of impinging currents, tidal scour, deposition of 
sediment, and the degradation of reefs by the numberless 
organisms that infest each block of coral, Mr. Murray 
depends more on the agency of solution, and on the distribu- 
tion of the food-supply. Those accustomed to balance evidence 
will be inclined to consider that the more probable explanation 
of the forms of reefs will be found in the association of the 
several agencies included in the explanations of these two 
naturalists. 
In November, 1882, Professor Alexander Agassiz published 
an important memoir on the Tortugas and the Florida reefs.* 
According to the American investigator, the atolis and barrier- 
reefs of the Florida Seas owe their form to the action of the 
breakers and currents, to the repressive influence of sediment, 
and to the habits of the corals. This opinion was the inde- 
pendent result of a line of investigation very similar to that 
which led Mr. Murray to his particular views. After alluding 
to the circumstance that the elder Agassiz had made no 
attempt to explain the substructure upon which the Florida 
reefs were based, this eminent naturalist proceeds to point 
out how the bottom is prepared and gradually raised to the 
levels in which reef-corals flourish by the accumulation of 
the solid parts of the numberless organisms that have lived 
and died upon it. On account of the increased supply of food 
brought by the Atlantic Equatorial current and by the Gulf 
Stream to the animals living on the sea-bottom in the Gulf 
of Mexico and in the Caribbean Sea, the deposits arising 
from the accumulations of the remains of molluses, echino- 
derms, alcyonarians, deep-sea corals, crustacea, &c., have 
been mainly heaped up in the track of these great oceanic 
currents. ‘There they have built up the great submarine 
plateaux, known as the Florida, Yucatan, and San Pedro 
Banks, on the surface of which the abundance of animal life 
is stated to be beyond our conception. A modern limestone, 
of which large portions were brought up in the dredge, is 
thus rapidly forming on these banks, which are in this manner 
being gradually raised to the level at which reef-corals 
flourish. In Yucatan, caverns penetrate this modern 
limestone, underlying the coral reefs to a depth of over 
400 feet. 
* Mem. Amer. Acad. Arts and Sciences. 
