66 THREE CRUISES OF THE " BLAKE." 



indeed rarely to be found below two thousand fathoms. The 

 thicker-shelled foraminifera reach a gr&ater depth, not because 

 they are of difPerent chemical composition, but because their 

 greater amount of substance yields less easily to the solvent 

 action of the acid or sea-water. At shallower depths the solvent 

 action of carbonic acid must be far less efficient, since there is a 

 rapid accumulation of dead siliceous and calcareous shells of 

 foraminifera, sponges, hydroids, corals, halcyonoids, mollusks, 

 polyzoa, echinoderms, etc., which must have lived upon the 

 bank long before they had by their accumulation brought it to 

 a level at which coral reefs could begin to grow. 



The bathymetrical sections (Figs. 40, 41) of the peninsula 

 of Florida to the eastward into the trough of the Gulf Stream 

 are very different from those taken on the western side of the 

 peninsula. Proceeding northward from Cape Florida, we pass 

 out of the action of the current from the Straits of Bemini, 

 where the velocity of the Gulf Stream is the greatest. As soon 

 as we reach a latitude at which the trade winds do not blow, we 

 come gradually upon the usual comparatively gentle slope off 

 shore, which shows little trace of disturbance either from currents 

 or from the action of the prevailing winds. Judging from what 

 I have seen of the east coast of the peninsula of Florida, the 

 shore line deposits, such as the coquina of St. Augustine and the 

 shelly beaches of Indian River, indicate the presence in deep 

 water of a limestone deposit formed of the detritus of mollusks, 

 annelids, starfishes, and sea-urchins. There are a few corals 

 only, and occasional patches of reef-builders, but no extensive 

 reefs ; but this is a difficult point to decide even at Key West. 

 Indeed, all along the line of the reefs it would be hard to deter- 

 mine to-day whether the reefs have been formed, like the Mar- 

 quesas Keys, merely by the accumulation of detrital matter 

 driven to the westward and northward, or whether the Mangrove 

 Keys and the reefs really indicate the old lines of a reef similar 

 to the one now in full activitv on the northern ed<re of the Gulf 

 Stream, parallel to the main line of keys. The absence of the 

 more delicate shells from limestone, in the formation of which 

 they must nevertheless have shared, may be explained by the 

 solvent action of carbonic acid upon them, and by the deposition 



