1870.] 59 [Pourtales. 



Tlie chief constituent is silicious sand from the coast line to about 

 the one hundred fathoms line, a limit which also coincides nearly with 

 the inner edge of the Gulf stream for a great portion of its course- 

 Outside of this line the whitish calcareous mud, also called Globige- 

 I rina mud, prevails and extends probably under the greater part of the 

 ocean. The silicious sand is replaced to the southward of the Vine- 

 yard Islands and off the eastern end of Long Island by a greenish or 

 bluish mud, known by the navigators as the Block Island Soundings. 

 Similar mud is found off Sandy Hook, in a range of depressions known 

 as the mud-holes, which form a leading mark to find the entrance of 

 New York in thick weather. In the neighborhood of New York a 

 few rocky patches are found, which require investigation. Near Cape 

 Fear, also, rocky bottom is sparingly found, affording a foothold to 

 some Corals, Gorgonians, and Sponges. Otherwise the sand is pretty 

 uniform in constitution, varying only in the size of the grain. 



A remarkable deposit of green sand is found on the inner edge of 

 the Gulf stream off the coasts of Georgia and South Carolina. The 

 bottom consists here chiefly of living or dead Foraminifera, the cham- 

 bers of the latter becoming filled with a silicate which injects even 

 the finest ramifications of the canals of the shell. At first yellow, it 

 gradually turns green, at the same time the shell proper decays and 

 breaks off, leaving a cast, which by attrition or conglomeration with 

 others often loses the characteristic form of a cast. Sometimes black 

 pebbles are found, of which a section shows plainly the origin, due to 

 an agglomeration of casts of Foraminifera. The dredgings made by 

 the Coast Survey in the Straits of Florida, have revealed the existence 

 of a large bank or deep sea plateau off the Florida reef, consisting of 

 a highly fossiliferous limestone, still in process of formation from the 

 numerous shells, Echinoderms and corals, mostly new to science, 

 which live on it at a depth of from one hundred to three hundred 

 fathoms. Between this plateau and the reef, the bottom consists of 

 the detritus of the reef, more or less finely comminuted, and not rich 

 in animal life. In depths beyond the three hundred fathoms line, but 

 with considerable variation in its limits, we find again the Globigerina 

 mud, which also fills the greater part of the Gulf of Mexico in deep 

 water. 



The Coast Survey intends to prosecute these researches next year 

 with increased means, 



