66 THREE CRUISES OF THE “ BLAKE.” 
indeed rarely to be found below two thousand fathoms. The 
thicker-shelled foraminifera reach a greater depth, not because 
they are of different 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, haleyonoids, mollusks, 
polyzoa, echinoderms, ete., 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, indieate 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 activity on the northern edge 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 
