SUBMARINE DEPOSITS 185 



There may be aU sizes of smaller stones forming various 

 kinds and. sizes of gravel, and grading down to coarse sand 

 and then to fine sand, and finally mud. The nature of 

 the sand and mud will depend upon the kind of rocks from 

 which it is derived or the sediments brought down by the 

 rivers ; and as a rule in most places the terrigenous deposits 

 become finer and finer the farther they are from the coast, 

 until the mud-line is reached, where the finest particles 

 suspended in the water are deposited. This is usually at 

 a depth of about 100 fathoms on continental shores facing 

 the open ocean. 



In addition to these shaUow-water sands and muds, 

 obviously derived from the adjacent land, and usually 

 characterized by quartz grains, Murray classifies under 

 terrigenous certain deeper muds coloured blue or red by 

 hydrated oxides of iron, or green by glauconite, and found 

 around continental lands, farther out and deeper than the 

 mud-line. 



There are also volcanic muds round oceanic islands of 

 volcanic origin and formed from the particles of volcanic 

 rocks. 



Around coral reefs and islands there may be coral-sands 

 and coral-muds, calcareous deposits formed of the frag- 

 ments of coral broken up and sometimes ground down to a 

 very fine powder. It is possible that some of these coral 

 muds are formed not mechanically but by bacterial action. 

 The late G. Harold Drew, working at Tortugas, Florida, 

 on the effect of Bacillus calcis in shallow tropical seas, found 

 that this organism caused the precipitation of soluble 

 calcium salts in the form of calcium carbonate (" drewite ") 

 on a large scale. He beHeved that his observations showed 

 that the great calcareous deposits of Florida and the 

 Bahamas, previously known as coral muds, are not, as was 

 supposed by Murray and others, derived from broken-up 

 corals, shells, nuUipores, etc., but are minute particles of 

 carbonate of lime which have been precipitated by the 



