FLORIDA REEFS. 37 
All the keys, all the islands along the whole extent of the reef, and all 
thfit part of the peninsula which consists of coral formations, have been 
formed by subaerial deposits accumulated under tidal agencies. The con- 
trast between these subaerial deposits and the rock of submarine origin is 
striking. Near the shore the torrential character of the stratification is still 
evident ; but at some distance from the islands and the main-land the strata 
are more regular, and contain, also, more and better-preserved fossil remains. 
The depth at which coral reefs start from some given foundation is stated 
by different authors to be Irom twelve to twenty fathoms. It would seem 
that the Florida reef has sprung up in somewhat shallower waters, for be- 
low ten or twelve fathoms there are no indications of living coral growth. 
The outer reef ends off tlie Marquesas in a depth of about ten fathoms. 
The ship-channel itself is only some ten fathoms in depth there, and very 
few coral heads are found in it. Add to this, that the depression between 
the Marquesas and Tortugas, though for the greater part not exceeding 
twelve or thirteen fjithoms, is entirely unobstructed by coral heads, and that 
in the ship-channel there are very few coral heads noticed below a depth of 
six or seven fathoms. We are therefore inclined to believe that, in lati- 
tudes bordering on the Tropics, the normal depth for the foundation of 
a coral reef is from ten to twelve fathoms, and that, if they spring up from 
greater depths, it must be in the Tropics proper. 
Plv/sical Changes in the Gulf Stream. 
There are several questions of the deepest scientific interest, which may 
be advanced by a due consideration of the facts observed upon the reefs of 
Florida. There we have a peninsula — a narrow, flat strip of land, project- 
insr for about five deo-rees from the main-land — between the Atlantic Ocean 
o o 
and the Gulf of Mexico, and forming an effective barrier between the waters 
of the two seas, which otherwise, even by the change of a few feet in the 
relative level of the intervening peninsula, would communicate freely with 
one another; and this peninsula we now know to have been added to the 
continent, step by step, in a southerly direction. 
We know that the time cannot be far behind us when the present reef, with 
its few keys, did not exist, and when the channel, therefore, was broader, 
and the Gulf Sti-eam flowed directly along the main range of keys. We 
know, further, that at some earlier period the keys themselves were not yet 
