CORALS. 131 



island. The first effect of the sinking island appears in the formation of a lagoon be- 

 tween the perijihery of the reef and the coast line. This lagoon grows in width as 

 the island sinks, while the constant growth of the coral on the outer rim keeps it at 

 the sea level. The submergence does not stoji until the top of the mountain sinks 

 below the waves, but even then the outer rim, the peripheral edge of the original coral 

 reef, still holds its place at the water's surface on account of the corresjjonding growth 

 of the corals at that point. The island has now a true atoll shape, a ring-shaped reef 

 of slight elevation above the sea level, with a diameter equal to that of the base of 

 the mountain when the submergence began, — the diameter, of course, measured be- 

 tween points which lie in the coralline zone of the island. In this way, yet more 

 graphically, Darwin has explained the circular form not only of true atolls but also of 

 many curved or crescentic islands, such as are very common in the Pacific and Indian 

 Oceans. 



Many objections have of late been urged against this theory of Darwin, and jirob- 

 ably other causes must be sought for to explain the circular outlines of many other 

 reefs which have the form of true atolls. Semper has suggested the direction of ocean 

 currents as an explanation of the circular reefs of the Pelew group. Let us see if a 



Fig. 123. ^ Islanil with flinging and barrier reel- 



similar cause cannot be found to accomit for the ring-shaped islands of the Atlantic 

 Ocean and Gulf of Mexico. 



Every one who studies a good niaji of the coral islands of southern Florida will 

 have his attention attracted to the general trend of a long series of islands extending 

 westward from Cape Florida into the Gulf of Mexico. He will perhaps notice that 

 these islands are very narrow in a north and south direction, and elongated from east 

 to west, strung along one after another in an almost direct, yet slightly curved line. 

 About midway in the chain, however, there are a few marked exceptions to this 

 general law, and these include some of the largest members of the grouj) known 

 as the Fine Islands. Upon one of these islands, called Key West, is situated a large 

 city of the same name. Unlike the other islands of the chain, tlie longer axes of the 

 Pine Islands, as pointed out by L. Agassiz, extend north and south, or at right angles 

 to the others. As a general rule, all the Florida Keys are low, formed of coral rock, 

 without caves of any size, and have no red soil, a characteristic of worn and eroded 

 coral islands. Their highest altitude, nowhere more than a few feet, is on their 

 southern border, and parallel with them throughout their whole course runs a coral 

 reef, sejiaratcd from them by the Hawk's Channel, which is a half-dozen miles broad in 

 its widest part. South of the reef, which is a succession of dangerous coral banks 



