THE FLORIDA REEFS. 73 
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 suff- 
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. 
