174 DEPTHS OF THE OCEAN chap. 



in the surface waters the Hving animals are as abundant over 

 the Red clay areas, where not a trace of their shells can be 

 detected in the deposits, as over the Pteropod ooze areas, where 

 every one of them may be found. 



At about 2500 fathoms the percentage of calcium carbonate 

 in the deposits apparently falls off more rapidly than at other 

 depths. In some areas, as, for example, in the North Pacific, 

 calcareous shells are not found in 2500 fathoms, while in 

 the North Atlantic they are at the same depth sufficiently 

 numerous for the deposit to be called a Globigerina ooze. 

 Where the living organisms are most numerous in the surface 

 waters, the dead shells are to be found at greater depths on the 

 ocean's floor than elsewhere. Where cold and warm currents 

 intermingle, shelled organisms are killed in large numbers, and 

 the dead shells may be found in deeper water than in neigh- 

 bouring regions. 



It must be remembered that while we know the crust of the 

 earth on the continental areas to the depth of several thousands 

 of feet, our knowledge of the crust under the oceanic areas is 

 limited to one or two feet. Only in a few exceptional instances 

 can we say that the sounding-tube has penetrated more than 

 eighteen inches or two feet into the deposit. Sometimes, when 

 the sounding-tube brings up a section over a foot in length, 

 there are distinct indications of stratification.^ Even in great 

 depths there may be a Globigerina ooze overlying a Red clay 

 in the deeper part of the section. This arrangement may be 

 explained by supposing that the calcareous shells have been 

 slowly dissolved from the deeper layers, but this explanation 

 will not suffice when a Red clay occupies the upper and a 

 Globigerina ooze the deeper layer of the section. This latter 

 arrangement appears to indicate that a large block of the earth's 

 crust may have subsided to the extent of several hundreds of 

 feet — from a depth at which a Globigerina ooze had been formed 

 in normal circumstances to a depth at which a Red clay is laid 

 down at the present time. 



There are not many cases on record of one type of deposit 

 being superposed upon another distinct type, examples being 

 more numerous of differences in colour and in composition in 

 the different layers of the same type of deposit. Thus, in Blue 



1 From his examination of the samples collected during the German South Polar Expedition 

 on board the " Gauss," Philippi believed that stratification on the sea-floor of to-day is not the 

 exception but the rule, and that, where it seems to be wanting, the upper layer is probably 

 thicker than the depth to which the sounding-tube penetrated. 



