131 



settling tube technique is comparable with analyses made by 

 screens, so that no difference should have resulted in the 

 sedimentary parameters from these practices. (3) There has 

 been an increase in the percentage of fine sediments, 

 especially silt and clay, since Shepard and MacDonald collected 

 their samples. It is known that the volvune of sediment 

 reaching the bay has decreased in historic time. It might be 

 reasonable to conclude that there has also been a decrease in 



grain size. In other continental shelves and submarine 

 regions where sediments were collected again after a lapse 

 of time there frequently are pronounced changes in texture. 

 In some areas where sediments have been collected only a 

 month apart the various sediment parameters may differ 

 so greatly that there is little similarity to be seen. 



Although an attempt was made to quantitatively measure 

 the rate of sedimentation by comparing Shepard and Mac- 

 Donald's samples with those collected by the Hancock 

 Foundation (Table V)9 the results were inconclusive because 

 it was difficult to accurately estimate the sediment thick- 

 ness represented by the samples. The only conclusion that 

 can be drawn from the two sets of samples is that most 

 of Santa Monica Shelf appears to be accumulating material 

 finer than prior to about 1934, 



