11. Rk.,rky. Rock; rocky; the naturally occurring material that 



forms the firm, hard, and solid masses of the ocean 

 floor. On navigation charts the abbreviation Rk. usually 

 indicates a rocky formation or a single large boulder 

 which may constitute a danger to navigation. 



12. Ck, Chalk; soft earthy limestone of marine origin composed 



chiefly of minute shells. It is white, gray, or buff in 

 color. Chalk is classed as rock on bottom sediment 

 charts. 



13. Qz. Quartz; silicon dioxide. The most connmon inorganic 



constituent of the detrital particles on the ocean floor. 



14. Co. Coral; the hard calcareous skeletons of certain tiny 



nnarine animcils, or the stony solidified mass of a 

 number of such skeletons. In warm waters colonial 

 coral forms extensive reefs of limestone. In cool 

 or cold water coral usually appears in the form of 

 isolated solitary individuals. Occasionally, large reefs 

 formed in cold waters by calcareous algae ( Litho- 

 thamnion) have been noted on navigation charts as coral. 



14a. Co. Hd. Coral head; a mass of coral, usually forming a portion 

 of a reef. Frequently, coral heads are large enough to 

 be dangerous to navigation. On the basis of size, coral 

 heads would be classed as rock, but are shown as coral 

 on bottom sediment charts, 



15. Mad. Madrepore; a branching coral; also any perforated 

 Md. stone coral. Madrepore is classed as coral on bottom 

 Mds. sediment charts, 



16. Vol. Volcanic. 



l6a. Vol. Volcanic ash; usually fine-grained material ejected by 



Ash a volcano. Volcanic ash is classed as sand on bottom 



sediment charts. 



17. La. Lava; fluid rock such as that which issues from a 



volcano or a fissure in the earth's surface; also the 

 same material solidified by cooling. Since this material 

 usually is nnassive, it is classed as rock on bottom 

 sediment charts. 



