492 BOUNDARIES OF THE SEA 



the conditions on earth, or whether they may have varied in 

 position, or Icind, and if so to what extent. Sediment cores are the 

 chief source of information on this historical question. 



The marine geologist can also attempt to act as intermediary 

 between the field geologist examining ancient continental ter- 

 races and the geophysicist studying recent ones. A fine example 

 of a well-exposed continental terrace with adjoining basin is 

 probably that of the late Paleozoic of western Texas, although 

 doubt has been expressed as to the validity of this comparison 

 (J. E, Sanders, personal communication). 



I put in a plea just now for more information on movements of 

 water in contact with the bottom. However, this point can be 

 reversed and sediments can be used as evidence to show what 

 movements are taking place. In this connection it is worth point- 

 ing out that sediments tend to record either maximum \^elocities 

 or average conditions for a locality, as the case may be, two 

 values which can usually not be ascertained by current measure- 

 ments. The occurrence of ripple marks, nondepositional conditions, 

 sandy strata between muds, and winnowed oozes are examples. 

 Marine chemists can learn from sediments whether lime is being 

 dissolved or precipitated in certain localities. 



Animal and plant remains in a sediment can give valuable in- 

 formation to biologists as to the limits of distribution and the 

 abundance of species. The connection between nutrients, plankton 

 blooms, the decay of organic matter, and sediments has been 

 mentioned. In fact, whenever a marine geologist has a question 

 for others to answer, he may be providing either data or at least 

 food for thought to the one being interrogated. Personally I have 

 found that attempting to answer a question is often more useful 

 to the one who replies than to the inquirer, partly because the 

 former is paying much better attention. 



Accumulation of Clay in Tidal Flats 



I want to treat in slightly greater detail a subject in which 

 practically all marine sciences meet on common ground, that of 

 the accumulation of fine material on tidal flats. This subject has 

 received much attention in northwestern Europe, and recent 

 advances made in Holland are worth recording. 



