282 THREE CRUISES OF THE " BLAKE." 



" In certain concretions found by the ' Blake ' in the Florida Straits, 

 and by the ' Challenger ' in vax-ious 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. 



" In 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 fracture. 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 browTi color, 

 consisting of an aggregation of calcareous organisms cemented by a 

 brownish yellow matter, often showing concentric rings after the man- 

 ner of agate.^ 



" At other stations small phosphatic concretions were also obtained 

 by the ' Blake,' all more or less resembling those described 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, Pourtales 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 livino-. 



The globigerina ooze of the Gulf extends northward until 

 it strikes the Mississippi River slojDe. 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, opbiurans, and sea-urchins, 

 characteristic of the continental Gulf slope, and tyj)ical 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 browu matter is iso- attacking the brown or yellow parts un- 



tropic ; between crossed nicols only the der the microscope with molybdate of 



calcite and the shells of the foraminifera anmionium and nitric acid, there is an 



V)righten up ; the calcite lies crystallized abundant yellow precipitate characteristic 



in the interior of the foraminifera. In of phosphoric acid." 



