MARINE DEPOSITS 221 



ocean, and also the great extension of lime- 

 secreting pelagic organisms in the tropical 

 surface currents flowing northwards and south- 

 wards from the equator. Solution of calcium 

 carbonate is continually going on in some 

 parts of a coral reef, while great deposition 

 by living organisms is proceeding in others. 

 In this way it is believed that the characteristic 

 form and features of barrier reefs and atolls 

 can be explained without calling in a sub- 

 sidence of the sea-floor, which for a long 

 time was considered an essential condition 

 for the formation of barrier reefs and atolls. 

 On the whole, the lime of the lithosphere is, 

 by the processes just indicated, being accumu- 

 lated towards the equator at the present 

 time. 



Phosphatic Nodules. — In certain regions 

 near land, where surface currents from 

 different sources alternate with the seasons, 

 phosphatic concretions, largely made up of 

 phosphate of lime derived from the decom- 

 position of organic remains, are accumulating 

 in considerable quantities on the underlying 

 sea-floor. 



Glauconite. — This is a silicate of potassiuni 

 and iron, the constituents of which are derived 

 from the decomposition of continental rocks 

 and minerals. It is generally deposited in the 

 internal chambers of foraminifera and other 



