NATURAL SCIENCE. 



and mineral particles too small for precise determination. These 

 fine washings do not make up a pure clay, but a substance allied to 

 bole-clay rich in iron and manganese. 



The I co-fathom contour is taken as the more or less arbitrary 

 boundary separating the deep-sea from the shallow water deposits. 

 This contour line divides the continental masses from the great 

 oceanic depressions. The area of the ocean between the shore and 

 the loo-fathom line is about ten millions of square miles, whereas 

 that between the loo-fathom and 500-fathom lines is only seven 

 millions. The area between the two last-mentioned contours may, 

 therefore, be spoken of as the continental slope. 



Marine deposits in general are classified as follows : — 



I Pelagic deposits formed 

 in deep water far away 

 from land. 



Deep-sea deposits beyond 

 100 fathoms. 



I Red clay. 

 Radiolarian ooze. 

 Diatom ooze. 

 Globigerina ooze. 



Blue mud. 

 Red mud. 

 Green mud. 

 Volcanic mud. 

 Coral mud. 



Terrigenous deposits 

 formed in deep and 

 shallow water close to 

 land masses. 



Shallow-water deposits he-\ 



tween low-water mark gands. gravels, muds. &c. 

 and 100 fathoms. J li 



Littoral deposits between] I 



high- and low - water [ Sands, gravels, muds. &c. 

 marks. j 



The following table indicates the average depth at which each 

 of the more important deposits is found, the average amount of 

 carbonate of lime present, and the area of distribution : — 



Red clay 

 Radiolarian ooze 

 Diatom ooze 

 Globigerina ooze 

 Pteropod ooze 



Coral mud 



Coral sand 

 Other terrigenous deposits 



Mean depth 

 in fathoms. 



2,730 



2,894 



1.477 



1,996 



1,044 

 7401 

 176/ 



1,016 



Mean per- 

 centage of 

 Ca CO3. 

 670 



401 



2296 



64-53 

 7926 



86-41 



19 20 



.^rea, 

 sq. miles. 



51,500,000 



2,290,400 



10,880,000 



49,520,000 



400,000 



2,556,800 



16,050,000 



It is with the deep-sea deposits alone that the authors of the 

 present volume are concerned. These cover more than one-half of 

 the earth's surface, and pass gradually into the shallow-water deposits. 

 Phenomena of erosion are almost entirely absent from the deep sea, 

 and the only mechanical actions of any consequence are those con- 

 nected with submarine volcanic eruptions. The effects of chemical 

 action, on the other hand, are strongly marked by the occurrence of 

 glauconite, phosphatic nodules, manganese nodules, and zeolites. 



