20 Proceedings of the Royal Physical Society, 



coast, first in a southerly direction, but gradually curving 

 round until it takes a due westerly trend. This westward 

 curve is attributed mainly to the influence of the strong 

 counter-current, which, with a width of 10 to 20 miles, 

 sweeps westward into the Gulf of Mexico along the left side 

 of the Grulf Stream, and heaps up organic debris in its track. 

 Florida is growing westward in the line of this current. Eeef 

 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 imme- 

 diately to the west of this group Professor Agassiz has found 

 a prominence on the submarine bank, on which corals have 

 begun to grow. Large heads of astrseans and madrepores 

 have fixed themselves at a depth of from six to seven fathoms, 

 and G-orgonim are found a little lower. This is the beginning 

 of an atoll. The Tortugas, which present a further stage of 

 development, consist of an elliptical, atoll-shaped reef, in 

 three chief parts, whereof the largest forms a crescent, front- 

 ing to the east, round the edge of the submarine bank, while 

 the two other portions have grown south-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 on the higher parts of the reef by the accumulation 

 and induration of calcareous debris tossed up on the reef by 

 the waves. To the breakers and currents, combined with 

 the distribution and habits of growth of the reef-builders. 

 Professor 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 

 Porites, occurring in clusters over the shallow tracts of coarse 

 sand, and Mceandrina areolata, growing between the marine 

 lawns of Thalassia, with occasional patches of Anadyomene. 

 Immense masses of nuUipores and corallines grow on the 

 tops of the dead branches of the madrepores, which have 

 been killed by exposure to the air during extreme low tides, 



