132 LOWER INVERTEBRATES. 



and small islands, the water becomes deep, forming a trough for a great oceanic 

 current, the Gulf Stream, whose floor at this point is called the Pourtalds Plateau. 

 South of this plateau is the deepest water of the stream which flows hard by the 

 neighborhood of Cuba. 



The cause of the general trend of the Florida Keys must be looked for in the 

 dii-ection of the Gulf Stream, or of a current flowing below it in an opposite 

 direction, of which they were once the northern bank. This " oceanic river " flows 

 tangentially to the Florida reefs throughout their whole length, and to it may be 

 traced the general trend of the Keys from the Tortugas to the southern extremity 

 of the peninsula of Florida. 



The exceptional position of the longer axes of the Pine Islands is directly due 

 to the Gulf Stream and tide currents about them. The submarine elevation or plateau, 

 upon the southern border of which the Florida Keys lie, has not the ^reat depth of 

 the Gulf of Mexico or the Gulf Stream bed between Florida and Cuba. On this 

 comparatively shallow platform the water rises in tides twice every twenty-four hours, 

 and at times, especially on its western extremity, the waters of the Gulf Stream are 

 forced over it. The water, thus raised above its natural level, must return to deeper 

 channels ; and one course which it may take is into the trough of the Gulf Stream 

 at right angles to the direction of its flow. The Pine Islands have their longer axes 

 tangential to several of these subordinate branches or currents. A similar phenomenon 

 is also seen in the channels which separate many of the Bahamas. The last but one 

 of the grouj)S of coral islands which compose the Florida chain, which is called the 

 Marquesas, has a circular or atoll-like shape. In this grouji we have a resultant of two 

 currents, or a combination of those minor currents which placed the axes of the Pine 

 Islands north and south and the Gulf Stream which gave the general trend to the 

 chain. From the position of the group near the extreme western end of the series 

 looking out into the depths of the Gulf of Mexico, these forces are difficult to separate, 

 and at intervals reinforce each other. The circular form of Marquesas is due to their 

 combined action. 



We have now arrived at a point in our discussion of coral islands where it 

 may be possible to appreciate the influence of ocean currents in the formation of atolls. 

 Let us consider the cluster of islands which occupies an irregular triangular space 

 midway between the southern point of Florida, the Bahamas and the island of Cuba. 

 This comparatively small coral bank is known as the Salt Key Bank, a coral plateau 

 which lies in the eddy of three great ocean currents. At most points the bank has a 

 moderate depth below the surface of the ocean, but in places along its outer rim there 

 are several coral islands of moderate size, some in process of formation, and others 

 which show the marks of great erosion. The Salt Key Bank, as far as it has been 

 explored, is a circular coral bank, fringed by islands which are not continuous above 

 water along its border, but which, nevertheless, are parts of a true atoll. On its sides 

 Salt Key Bank is washed by ocean currents, on the north by the Gulf Stream, to 

 which the Florida Keys owe their formation, on the east and south to the Bahama 

 current in the old Bahama channel. Coral islands, as pointed out by Semper, form 

 tangentially to the direction of ocean currents, and the outlines of the Salt Key Bank 

 result from the direction of the three currents which surround it. 



There are few evidences of submergence in the Salt Key plateau, and if we cross 

 the Bahama channel to Cuba we find terraced coral banks elevated above the sea, 

 showing a great elevation of the coasts. The Salt Key bank, as well as the Florida 



