214 THE SEA FLOOR 



founded on the dominant element or characteristic 

 aspect of each type of deposit, and attempts to bring 

 into prominence their origin and constitution. There 

 is no sharp line of demarcation between the different 

 types ; they all shade the one into the other, and it is 

 often difficult to determine what name should be given 

 to a deposit on the border line between two or three 

 types.* For the information of the geologist, physical 



* It is essential to be continually on one's guard against 

 sources of error when examining deep-sea deposits. The 

 object in view is to arrive at a true notion of the composition 

 of the deposit as it lies at the bottom of the sea. Sometimes 

 the trawl or dredge may come up filled with fully a ton of 

 manganese nodules, or large and small fragments of pumice ; 

 there may be hundreds of sharks' teeth mixed with dozens of 

 ear-bones of whales. Some of these may have living Lepas 

 or Hydroids, or Serpula, or Foraminifera, attached to them. 

 It has sometimes been supposed that the whole deposit was 

 composed of these remains ; it is, however, almost certain 

 that these various objects were embedded in clay, which 

 passed through the meshes of the nets. The same may be 

 observed with reference to the phosphatic and glauconitic 

 nodules found in relatively deep water along continental 

 shores. In using sounding tubes with butterfly valves, a 

 large proportion of the finer parts of the deposit is sometimes 

 washed out while the tube is being hauled in, and the larger 

 calcareous organisms appear to be more abundant than they 

 really are. When a large quantity of deposit is brought up 

 in the dredge or trawl, and passed through sieves, the various 

 constituents are separated, and the different portions have 

 occasionally been described separately ; in this way have 

 arisen erroneously the names Orbulina ooze, Biloculina clay, 

 and Coprolitic mud. In like manner the finer parts of a 

 Radiolarian ooze have been described as Diatom ooze, and 

 the coarser parts of a Diatom ooze as Radiolarian ooze. 

 When the deposit is preserved in water or spirit, the sample 

 is sorted into different layers by shaking ; if a portion be taken 

 for description from the upper layers, and another from the 

 lower layers in the bottle, widely different results will be 

 obtained. 



The best sample for description is that which is taken 



