xi The Bed of the Oceans 203 



which drop land -derived stones and mud all along the 

 path of the ocean currents, which drift them into warm 

 seas. Wind also blows sand or dust far out to sea. Volcanic 

 eruptions throw up quantities of fine dust, which are 

 carried far and wide by the winds and scattered over the 

 whole sea surface. Pumice-stone, being porous, floats for 

 months and probably years, and may be drifted to any part 

 of the ocean before it becomes waterlogged and sinks. All 

 terrigenous deposits, although soft and sticky when wet, 

 fall int'o a loose powder on being dried, hence the term Mud 

 is specially applied to them. Such deposits are characteristic 

 of enclosed seas and of the upper margin of the Transitional 

 Area, which they clothe much as snow clothes a tropical 

 mountain, most thickly on the upper part of the slope. It is 

 estimated that terrigenous deposits cover one-fifth of the 

 area of the oceans, and it is distinctive of these deposits 

 that they are made up of fragments of continental rocks, 

 such as compact limestone, quartz, schist and gneiss. 



270. Blue Mud. The littoral deposits or shore forma- 

 tions sometimes extend in the form of sand or bars of 

 fine gravel, enclosing hollows filled with mud, right across 

 shallow seas. As a rule, however, deep enclosed seas, 

 margins of islands and of continents for 200 or 300 miles 

 from land, are carpeted with extremely fine mud, containing 

 small grains of sand and the remains of shells and of 

 marine plants. Where the material is derived mainly from 

 rivers it assumes the form of a blue mud, which is the most 

 characteristic of terrigenous deposits in every ocean, and is 

 found at all depths. Blue mud owes its dark blue or slaty 

 colour to chemical changes produced by decomposing vege- 

 table and animal substances, in presence of the sulphates of 

 sea- water, which appear to be reduced to sulphides, and 

 decompose the ferric oxide abounding in all deposits into 

 sulphide of iron and ferrous oxide. When there is much 

 iron in the state of ferric oxide, as in the ochrey muds that 

 redden the water of the Amazon, there may not be sufficient 

 organic matter to reduce it all, and the mud retains its red 

 colour. Blue mud contains variable quantities of carbonate 

 of lime according to the abundance of shell-producing 



