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 alonjj the 

 shores of the Gulf, where rivers enter and where there are few coi-al 

 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 foramiuifera as 

 well as other calcareous organisms. Where the shores are lined by 

 coral reefs, the deposits are chiefly made up of coral debris, 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 glauconitic particles were occa- 

 sionally noticed. The mineral particles and fine clayey matter appear 

 to be almost wholly derived from the rivers emptying into the Gidf of 

 Mexico. 



" The carbonate of lime in the deposits of these regions is mostly 

 made up of the shells of pelagic foraminifera and mollusks, though we 

 also find coccoliths, rhabdoliths, and fragments of echiuoderms. In 

 depths greater than 2,000 fathoms, the pteropod and heteropod shells 

 api^ear to be nearly, if not quite, absent, — the cai'bonate 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 

 volcanic muds. In the coral muds it varies from sixty-seven to 



