CORALS AND CORAL ARCHITECTURE. 267 



the coral structure. In many of the branching, tree-like corals the 

 stems are formed nearly solid as they grow, and are of great strength. 

 In some of the massive species the surface cells occupied by the living 

 animals are very shallow, measuring from one-sixteenth to one-fourth 

 of an inch in depth. Underneath the polype is a floor or partition of 

 coral secreted by the animal, and which separates the new from the old 

 cell. Hence many corals, when split vertically, show a coarse cellular 

 structure. In the life-and-death process of the polypes, animal matters 

 remain confined in the old cells. 



As the coral masses increase in size, it is evident that there must 

 have occurred a simultaneous increase in the number of polypes. This 

 fact is the more interesting, when it is known that a great coral dome 

 may have arisen from a few, or perhaps from a single parent indi- 

 vidual. During the long period of its growth, reaching through thou- 

 sands of years, how enormous is the number of builders of it that have 

 lived and died ! 



The rate at which corals grow is an interesting question, but not 

 fully determined, for want of sufficient data. A single mass standing 

 in clear water would increase more rapidly than corals in a reef. If 

 at the rate of an inch in six years, a dome 20 feet in diameter would 

 require about 1,400 years. Some species seem to grow more rapidly 

 than this, but the increase of reefs is slower, notwithstanding addi- 

 tions from shells and other sources. On a coral-plantation, as a reef 

 may be called, a portion is always unproductive. There ai'e barren 

 areas on the reef where sands or sediment destroy the polypes, and retard 

 its growth. The investigations of Prof. Agassiz, at Key West, indi- 

 cate a growth of about six inches in 100 years. He says : " If we allow 

 twice that rate of growth, not less than 7,000 years would be required 

 for the formation of the great reef at that place, and hundreds of 

 thousands of years for the coral growths which form the peninsula of 

 Florida." 



After a careful estimate, Prof. Dana concludes that the growth of 

 reefs, from increase of their corals, may be from ^ to -^ of an inch 

 per year, and adds that, " whatever the uncertainties of calculation, 

 is is evident that a reef increases with extreme slowness." It is a 

 reasonable calculation that more than 1,000,000 years have elapsed 

 since the foundations were laid of some of the great Pacific reefs. 



An opinion prevailed formerly that the different species of corals 

 occur in a reef in a uniform order of superposition that each flourishes 

 at certain depths of water, and not above or below that plane. The 

 general fact is known that no important reef-building coral grows at a 

 depth greater than 120 feet. Above that plane, all the work of coral 

 architecture is carried on. Prof. Agassiz supposes that the range of 

 different corals in depth is in part limited by pressure of the waters. 

 At 32 feet depth, the animal is under a pressure of two atmospheres, 

 and of more than four atmospheres at 120 feet. In this connection he 



