SECONDARY PRODUCTS 211 



5. In addition to the foregoing there are many sub- 

 stances which may be termed " chemical " or " secon- 

 dary " products. Clayey matter, which arises from 

 the decomposition of pumice, felspars, and other 

 minerals on the sea floor, may be regarded as a 

 secondary product ; so also may the manganese nodules 

 so abundant in some Red Clay areas, where the lapilli 

 and bombs of basic rocks thrown out by volcanoes 

 abound ; the palagonite and zeolitic materials found in 

 the Red Clay may be placed in the same category. In 

 addition, "greensand" (glauconite) grains, glauconlric 

 casts of calcareous organisms, phosphatic concretions, 

 sulphate of barium nodules, iron and calcareous con- 

 cretions, must be classed among the secondary products 

 now forming in the deposits on the sea bed, many of 

 them through the intervention of decaying organic 

 matter and bacteria. 



The varied materials just enumerated are distributed 

 very unequally over the floor of the ocean. The 

 terrigenous materials are most abundant along the 

 shores of continents and islands, and in all enclosed and 

 partially enclosed seas. The geological structure of 

 the adjacent land-masses determines the general 

 character of the deposits along the shores and in the 

 shallower waters, especially as regards the inorganic 

 constituents. The deposits off coral-reef shores are, 

 of course, chiefly made up of fragmentary particles 

 derived from the growing reefs. The size of the 

 fragments diminishes, as a rule, as the distance from 

 the shore line and the depth of water increase ; but this 

 is not always the case, for in the very centre of the 

 Pacific and in very deep water rounded fragments of 

 pumice have been obtained of all sizes, up to more than 

 i foot in diameter. Again, in regions now, or in 

 recent geological times, affected bv floating ice, large 



