282 THREE CRUISES OF THE “BLAKE.” 
* In certain coneretions found by the * Blake’ in the Florida Straits, 
and by the ‘Challenger’ in various parts of the world, near land, the 
quantity of phosphate of lime is very much greater. These concre- 
tions appear always to be associated in an intimate way with or- 
ganisms. 
“Tn 125 fathoms, southwest of Sand Key, a fragment of manatee 
bone was obtained several centimetres in diameter. It was of a dirty 
brown color, of great hardness, and had a conchoidal siacture. A mi- 
croscopic examination of thin sections showed that the bone structure 
was perfectly preserved. It was found to contain over thirty-three per 
cent of phosphoric acid, and nearly fifty-two per cent of lime. 
“ From the same place was obtained a concretion of a brown color, 
consisting of an aggregation of ealeareous organisms cemented by a 
brownish yellow matter, often showing concentric rings after the man- 
ner of agate.! 
* At other stations small phosphatie coneretions were also obtained 
by the ‘Blake,’ all more or less resembling those deseribed above. 
There are difficulties in understanding how phosphate of lime and car- 
bonate of lime are deposited at the bottom of the sea, yet there is no 
doubt that such a deposition does take place under some special cir- 
cumstances. Their solution is, however, an almost universal phenome- 
non in the ocean." 
When dredging off Cuba, Pourtalds procured, at the depth 
of 270 fathoms, a number of nodules of a very porous lime- 
stone, similar in color and texture to the limestone which forms 
the range of low hills on the shore of Cuba, and. is composed 
of the remains of the same animals which are found living. 
The globigerina ooze of the Gulf extends northward until 
it strikes the Mississippi River slope. Here the fauna changes, 
and with the presence of dark rich muds that of the deep 
Gulf ooze disappears, and is replaced by a number of interest- 
ing forms of annelids, mollusks, ophiurans, and sea-urchins, 
charaeteristie of thé continental Gulf slope, and typical of mud 
deposits. Nowhere around the shores of the United States 
can we so readily trace the denudation of the continent as 
in the succession of formations which since the time of the 
! * This yellowish brown matter is iso- attacking the brown or yellow parts un- 
tropic ; between crossed nicols only the der the microscope with molybdate of 
caleite and the shells of the foraminifera ammonium and nitric acid, there ig an 
brighten. up ; the calcite lies crystallized abundant yellow precipitate characteristic 
in the interior of the foraminifera. In of phosphoric acid.” 
