^ec. 6, 1883] 



JVA TURE 



125 



bearing ocenn currents. They flourish on the windward 

 side of the islands and along the whole eastern coast of 

 Honduras, Venezuela, and S'ucatan. But on the leeward 

 shores they do not exist at all. Cuba is fringed both on 

 the north and south side with reefs, but the southern reefs, 

 directly bathed by the Gulf Stream and exposed to the 

 prevailing winds, are more flourishing than the northern 

 reefs, which are to some extent cut off from the equa- 

 torial current by banks and islands. 



The depth at which corals will flourish in these seas 

 has been found to be rather less than that which has been 

 ascertained to be in general their downward limit else- 

 where. Prof Agassiz concludes that they do not thrive 

 below a depth of six or seven fathoms in the Florida 

 seas, though on the outer reef, directly exposed to the 

 open currents and prevalent winds, they descend in 

 scattered heads to about ten fathoms. 



Each successive stage in the growth of an atoll seems 

 to be laid open for study in the prolongation of the 

 Florida reefs. The map of that region (Fig. 2) shows a 

 remarkable broken line of islets and strips of land running 

 parallel with the coast, first in a southerly direction, but 



gradually curving round until it takes a due westerly 

 trend. This we-tward curve is attributed mainly to the 

 influence of the strong counter-current which, with a 

 width of ten to twenty iniles, sweeps westward into the 

 Gulf of Mexico along the left side of the Gulf Stream, 

 and heaps up organic debris in its track. Florida is 

 growing westward in the line of this current. Reef after 

 reef is added to the land at the east end, while towards 

 the west, new reefs successively begin on the bank, as its 

 surface is gradually built up by the accumulation of 

 organic debris. 



The last and youngest of the reefs marked on the maps 

 and charts is the group known as the Tortugas. But 

 immediately to the west of this group Prof Agassiz has 

 found a prominence on the submarine bank, on which 

 corals have begun to grow. Large heads of Astr;eans 

 and Madrepores have fixed themselves at a depth of 

 frotn six to seven fathoms, and Gorgonii"e are found a 

 little lower. This is the beginning of an atoll. The Tor- 

 tugas, which present a further stage of development, con- 

 sist of an elliptical, atoll-shaped reef, in three chief parts, 

 whereof the largest forms a crescent, fronting to the east 



GULF OF MEXICO 



■ iu ■■■•i;;:-vi.-:Kpv<i- ■ 



Fig. 2.-Map of ihe Florida Reef and Kcj^ 



round the edge of the submarine bank, while the two 

 other portions have grown souih-westwards along the 

 bank. Three channels between these portions allow 

 powerful tidal currents to rush across the central chiefly 

 submerged parts of the atoll. Seven islands have been 

 formed at the higher parts of the reef by the accumula- 

 tion and induration of calcareous debris tossed up on the 

 reef by the waves. ' To the breakers and currents com- 

 bined with the distribution and habits of growth of the 

 reef-builders Prof. Agassiz entirely attributes the form 

 and growth of the reef The most important corals are 

 the Madrepores, which flourish in extensive patches, two 

 common species of Pontes occurring in clusters over the 

 shallow tracts of coarse sand, and MiTandritta areolata, 

 growing between the marine lawns of Tha/assiu, with 

 occasional patches of AnadYoinene. Immense masses of 

 nullipores and corallines grow on the tops of the dead 

 branches of Madrepores which have been killed by ex- 

 posure to the air during extreme low tides or when strong 

 winds have blown the water off the flats. Large heads of 

 Astrafans and Ma-andrin-i occur here and there towards 

 the edge of the reef, which is occupied mainly by clusters 

 of GorgoiiJiT. The destruction of the reefs by the waves 



is very great, the sea being occasionally discoloured with 

 the chalky sediment to a distance of from six to ten miles 

 after a storm. Broken coral-heads, and branches, dead 

 corallines, shells of mollusks, old serpula; tubes, stalks of 

 Corgoiii\r, and other organisms are throw'n up in lines 

 that consolidate into a low dyke, which in turn is pounded 

 up and removed by the breakers. A prodigious quantity 

 of calcareous sediinent is thus produced, much of which 

 is swept into the interior of the reefs, where it accumu- 

 lates in flats of sand and silt. It is only on the outer 

 edges of the reef, where the scour of the sea is greatest, 

 that the corals can flourish ; elsewhere they are choked 

 and buried under the deposit of calcareous sediment. 

 Some of this sediment accumulates in steep submarine 

 banks, like sand-dunes, which shift to and fro as winds 

 and currents vary ; though by the action of the carbonic 

 acid of the sea-water they are apt to be cemented into 

 solid slopes, some of which have an angle of as much as 

 33". So great is the destructive and transporting influence 

 of the sea under the combined or antagonistic working of 

 tides, currents, and wind-waves, that the whole mass of the 

 reef as well as the flats and shoals inside may be said to 

 be in more or less active movement. Hence none of the 



