280 THREE CRUISES OF THE “ BLAKE.” 
with the “Blake,” very extensive series of bottom deposits 
were obtained by the vessels of the Coast Survey, at all depths 
and in all parts of the Gulf of Mexico. Of these deposits 
Murray says in his Report : — 
“ There is a very great variety in the shallow-water deposits under 
one hundred fathoms. Near the coasts of the continent along the 
shores of the Gulf, where rivers enter and where there are few coral 
reefs, the deposits are either sands or fine clayey muds, formed of de- 
trital matter brought down from the land. We find in these muds, 
however, in the track of currents, a number of pelagic foraminifera as 
well as other calcareous organisms. Where the shores are lined by 
coral reefs, the deposits are chiefly made up of coral débris, the shells 
of pelagic foraminifera and mollusks, and other calcareous organisms. 
“The character of the deposits in depths greater than one hundred 
fathoms is likewise largely determined by the greater or less proximity 
to coral reefs or the embouchure of rivers. 
* In all the deeper deposits in the Gulf of Mexico and the Straits of 
Florida, the crystalline mineral particles are very small, rarely exceed- 
ing one tenth of a millimeter in diameter. "They consist principally of 
small rounded grains of quartz, with fragments of feldspar, mica, 
hornblende, augite, magnetite, and rarely tourmaline. In a few places 
there were fragments of pumice, and glauconitie particles were occa- 
sionally noticed. The mineral particles and fine clayey matter appear 
to be almost wholly derived from the rivers emptying into the Gulf of 
Mexico. 
“ The carbonate of lime in the deposits of these regions is mostly 
made up of the shells of. pelagie foraminifera and mollusks, though we 
also find coccoliths, rhabdoliths, and fragments of echinoderms. In 
depths greater than 2,000 fathoms, the pteropod and heteropod shells 
appear to be nearly, if not quite, absent, — the carbonate of lime then 
consisting of the shells of pelagic foraminifera; in lesser depths the 
pteropod and heteropod shells are present; and in depths of 200 
to 500 fathoms they make up the bulk of the deposits in many places. 
In several of the deposits, where the percentage of lime is very high, 
the whole has a very chalk-like appearance ; it appears, indeed, as if it 
were in the process of transformation to true chalk.” 
According to Murray, in the deposits of the Gulf of Mexico 
the carbonate of lime varies greatly. In muds brought down 
by rivers, there is sometimes little more than two per cent of lime. 
The percentage rises to ten or fifteen per cent in some of the 
voleanie muds. In the coral muds it varies from sixty-seven to 
