CONDITION OF THE FLOOE OF THE OCEAN. 317 



In tropical regions removed from land, where the depths are less than 

 600 fathoms, the carbonate of lime due to the remains of these organisms 

 from the surface may rise to 80 or 90 per cent; with increase of depth, 

 and under the same surface conditions, the percentage of carbonate of 

 lime slowly diminishes, till, at depths of about 2,000 fathoms, the 

 average percentage falls to about 60, at 2,400 fathoms to about 30, and 

 at about 2,600 fathoms to about 10, beyond which depth there may be 

 only traces of carbonate of lime due to the presence of surface shells. 

 The thin and more delicate surface shells first disappear from the 

 deposits; the thicker and denser ones alone persist to greater depths. 

 A careful examination of a large number of observations shows that 

 the percentage of carbonate of lime in the deposits falls off much more 

 rapidly at depths between 2,200 and 2,500 fathoms than at other depths. 



The red clay which occurs in all the deeper stretches of the ocean 

 far from land, and covers nearly half of the whole sea floor, contains — 

 in addition to volcanic debris, clayey matter, the oxides of iron and 

 manganese — numerous remains of whales, sharks, and other fishes, 

 together with zeolitic crystals, manganese nodules, and minute mag- 

 netic spherules, which are believed to have a cosmic origin. One haul 

 of a small trawl in the central Pacific brought to the surface on one 

 occasion, from a depth of about 2^ miles, many bushels of manganese 

 nodules, along with 1,500 sharks' teeth, over 50 fragments of earbones 

 and other bones of whales. Some of these organic remains, such as 

 the Carcharodon and Lamna teeth and the bones of the Ziphioid 

 whales, belong apparently to extinct species. One or two of these 

 sharks' teeth, earbones, or cosmic spherules may be occasionally 

 found in a globigerina ooze, but their occurrence in this or any deposits 

 other than red clay is extremely rare. 



Our knowledge of the marine deposits is limited to the superficial 

 laj^ers; as a rule the sounding tube does not penetrate more than 

 6 or 8 inches, but in some positions the sounding tube and dredge have 

 been known to sink fully 2 feet into the deposit. Sometimes a red 

 clay is overlaid by a globigerina ooze, more frequently a red clay over- 

 lies a globigerina ooze, the transition between the two layers being 

 either abrupt or gradual. In some positions it is possible to account 

 for these layers by referring them to changes in the condition of the 

 surface waters, but in other situations it seems necessary to call in ele- 

 vations and subsidences of the sea floor. 



If the whole of the carbonate of lime shells be removed by dilute 

 acid from a typical sample of globigerina ooze, the inorganic residue 

 left behind is quite similar in composition to a typical red clay. This 

 suggests that possibly, owing to some h3'pogene action, such as the 

 escape of carbonic acid through the sea floor, a deposit that once was a 

 globigerina ooze might be slowl}" converted into a red clay. However, 

 this is not the interpretation which commends itself after an examina- 



