MODERN SEDIMENTS 403 



6. What changes has the sediment undergone after its deposition and 

 what are the causes of those changes ? 



If all these questions regarding an}- one geologic formation could be 

 answered fully, we should have made a considerable advance in inter- 

 preting the histor}' of the earth, for such information could be applied 

 in interpreting other formations; but it is safe to say that very few of 

 the questions can be answered for any formation; it is also safe to say 

 that some of the questions can not now be answered for any geologic 

 formation, because the answers depend on deductions from premises that 

 must be. but have not yet been, inductively established. In the investi- 

 gation of sucli subjects as sediments the general metliod of modern science 

 must be followed — that is, we must first build premises inductively, and 

 then by deduction from those premises interpret phenomena that can not 

 now be observed. 



Modern Sediments 



In order to understand sediments, both modern and ancient must be 

 studied, but a reliable basis for interpreting ancient sediments can be 

 obtained only through a study of sediments now in process of formation. 



Researches on modern sediments should cover the following kinds : 



1. Continental deposits. These embrace (a) colluvial, fluvial, lacus- 

 trine, spring, and progradation deposits in cold, temperate, and torrid 

 climates of humid regions; (h) eolian, talus slope, and water-laid de- 

 posits in desert plain and mountainous regions and in desert basins ; 

 clastic, organic, and chemical deposits in inclosed lakes; (c) glacial 

 deposits. 



2. Great lake deposits. 



3. Marine deposits, (a) Deposits, mostly detrital, formed under di- 

 verse conditions, at the edges of coastal plains and off steep shores, as 

 follows : In protuberant deltas, in bays, and in areas between embay- 

 meni'5, in each major climatic zone; (h) deposits, mostly organic and 

 chemical, on submarine plateaus, in lagoons, and on* reefs, in tropical and 

 subtropical waters; (c) the origin and distribution of the deeper-water 

 marine sediments. A few subjects of importance in the study of marine 

 sediments are the relations between certain abyssal deposits in the ocean 

 (particularly red clay and manganese nodules) and the physico-chemical 

 condition of ocean water; changes in outline and position of shorelines; 

 direction of the movement of littoral drift; influence of depth, distance 

 from shore, etcetera, in determining the character and continuity or lack 

 of continuity of deposits; the effect of different degrees of salinity of the 



