THE FLORIDA REEFS. 78 



While on the way from Key West to the Tortugas we stopped 

 at the Marquesas/ which are grouped as a circular ring of 

 islands. Their formation has undoubtedly been identical with 

 that of the great Alacran Reef; and from the fact that no corals 

 are now found living on their weather side, these islands must 

 have assumed their present shape at the time when their weather 

 side made a part of the outer reef in connection with the islands 

 of Key West and the other keys, previous to the formation of 

 the present growing reef, or while the latter existed only in the 

 shape of a submerged reef several fathoms below the surface. 

 The plan and section of the Marquesas Keys (Fig. 44) show 

 the formation of the keys on a knoll rising from the general 

 platform of the surrounding reef plateau. This knoll has un- 

 doubtedly been built up, as were the Tortugas, from the remains 

 of the corals which once lived upon its face and surface until 

 the formation of the outer reef shut out the prevailing easterly 

 winds, and the corals were killed through the accumulation of 

 silt upon them. 



The filling of a lagoon like that of the Marquesas must be a 

 slow process, for we find the water of the inner lagoon deeper 

 than that of any part of the reef immediately surrounding the 

 outer slope. We can imagine that when the outer ring of the 

 reef surrounding the inside lagoon is once completed, or nearly 

 so, the enclosed calm area is so placed as to be subject to but few 

 disturbing agencies, and is practically excluded from receiving 

 any appreciable amount of sediment from the water of the outer 

 reef, since the lagoon connects with the surrounding waters only 

 by the narrow passages which form the channels between the 

 lagoon and the main channel. Whether the removal of the 

 dead coral rock from the interior of the lagoon of an atoll, by 

 the action of the current through the narrow connecting chan- 

 nels, and by the solvent action of the carbonic acid, will suffi- 

 ciently explain the great depth of the interior lagoon seems 

 somewhat doubtful. The mud of the interior of the Marquesas 

 atoll was found to be calcareous, as is practically all the mud 

 which forms the extensive mud flats to the northward of the 

 keys. This mud is, however, generally covered by a thin dark- 



1 See Bull. M. C. Z., V. No. 6. Letter No. 2, July, 1878. 



