18. Pm. Pumice; an excessively cellular, glassy lava. It is very 



light and floats on water until it becomes water logged 

 and sinks. Pumice is classed as gravel on bottom 

 sediment charts. 



19. T. Tufa; a chemical sedimentary rock composed of calcium 



carbonate or silica. Tufa is included with rock on bottom 

 sediment charts. On U. S. Navy Hydrographic charts for 

 the western coast of France the notation T may mean 

 tangue, a calcareous deposit derived from a variety of 

 marine organisms. 



20. Sc. Scoriae; volcanic slag, pyroclastic ejecta; fragments 



of scoriae between 0.16 and 1.26 inches (4 and 32 

 millimeters) in size are essentially equivalent to 

 volcanic cinders. Scoriae are classed as gravel on 

 bottom sediment charts. 



21. Cn. Cinders; volcanic cinders (see scoriae). 



22. Mn. Manganese; manganese dioxide; the black oxide of a 



metal. Manganese dioxide usually is found on the ocean 

 bottom in the form of nodules. Manganese nodules are 

 classed as gravel on bottom sediment charts. 



23. Sh. Shell; the hard outer covering of an animal. Shell 



usueQly falls within the gravel size limits, but because 

 of its origin shell usually is classed separately on 

 bottom sediment charts. 



24. Oys. Oysters; sessile moUusks with an unequal, irregular 



bivalved shell. Oysters frequently occur in large banks. 

 Oysters are classed as shell on bottom sediment charts. 



25. Ms. Mussels; sessile moUusks with a regular bivalved 



shell. Mussels frequently occur in large banks. Mussels 

 are classed as shell on bottom sedinnent charts. 



26. Spg. Sponge; a Porifera, a sessile invertebrate with a 



skeleton formed of a mass of elastic fibers (spongin) 

 and spicules. 



