3845 



HYDROGRAPHIC MANUAL 



Page 304 



comprises most of the sediments on the continental shelves and slopes, and also occurs in parts of the 

 ocean deeps adjacent to land. 



b. Organic material. — Remains of animals and plants, common in many areas near land. 



c. Volcanic material. — Debris from volcanoes, rare except in volcanic regions. 



d. Chemical material. — Material deposited chemically or formed on the sea floor by chemical 

 action. Included are phosphatic nodules, manganese nodules, and glauconite. The last is common 

 in many areas near land. 



3845. Classification of Sediments 



The basis for the classification of sediments is the size of the particles composing; 

 them. For a precise typing a mechanical analysis is necessary, but this is ordinarily 

 impracticable in hydrographic surveying. 



Sediments may be generally classified into three main groups: (a) sands, (6) those 

 finer-grained than sand, and (c) those coarser than sand. 



Sand is generally easily recognized; even the finer-grained sands feel gritty when 

 rubbed with the finger on the palm of the hand, and if scattered when dry, will separate 

 into grains easily visible to the naked eye. A\^ien mixed with water in a test tube it 

 sinks rapidly, usually in less than 1 minute, and is to a noticeable degree conductive of 

 water. 



Technically there are two grades of sediment finer than sand. These are silt and 

 clay. Because of the frequent difficulty in distinguishing them without special appara- 

 tus, for practical purposes they are often both classified under the general term mud. 

 "When the grains are finer than 0.05 imn in diameter, they have lost the physical proper- 

 ties of sand. The specimen no longer feels gritty when rubbed in the fingers, and if 

 dropped on a hard, level surface, the grains will not separate, but will congregate into 

 small heaps. Several minutes are required for the particles to sink in water to the bot- 

 tom of a test tube. Clay is a finer-grained deposit than silt. Sediments composed of 

 clay are stiff in nature and sticky to the touch. Ooze is very soft or ooze-like. 



Sediments are variously typed according to the size of the particles composing 

 them. There is no exact agreement among scientists as to classification but the follow- 

 ing tabulation will serve as a gage for classification of the sands and coarser particles. 

 It is not intended that the dimensions be measured. A careful estimation by eye is 

 satisfactory. Sand grains are easily visible and their relative coarseness can be closely 

 estimated with a little practice. 



Table 7. — Sediments classified by size 



