IXADEQIACY OF SOME EARLIER INVESTIGATIONS 405 



with the result that very nearly all particles below a certain size have 

 been removed. Knowledge of the transporting power of water of a cer- 

 tain density at certain rates of motion is necessary for understanding the 

 sizing of the deposit. The part played by other factors, physical and 

 physico-chemical, also needs to be known. 



Mechani'cal analyses of deposits are indispensable in the study of sedi- 

 ments, but they alone will not supply all the information needed for the 

 interpretation of the history of any deposit. The same may be said of 

 other physical features of sediments, such as shapes of particles, relative 

 amount of pore space, size of pores, and total surface of particles per unit 

 volume, all of which is necessary information. These physical determi- 

 nations do not take into consideration other factors, chemical and bio- 

 logic, that have played roles in the origin of most sediments. 



Very valuable researches have been conducted on the chemistry and 

 physical chemistry of sediments and sedimentary processes, but they do 

 not solve the problem of the origin of any sedimentary formation. Clarke 

 and Wheeler have shown that the difference in the chemical composition 

 of the skeletons of marine invertebrates and calcareous algae is the deter- 

 mining factor of the difference in the chemical composition of certain 

 marine sediments, but Professor Clarke himself emphasizes, in conversa- 

 tion, the inadequacy of such analyses alone to solve problems of sedi- 

 mentation. With reference to investigations in physical chemistry: 

 Should we know precisely the relations of CO2 in sea-water to depth, 

 temperature, and other physical factors in the ocean, and to the CO2 

 content of the air above the ocean, we should not solve the problems pre- 

 sented by any sediment, but without such knowledge the origin of certain 

 deposits can not be understood. The reason of the failure of this infor- 

 mation to solve the problem is because it concerns only one of a number 

 of factors involved in producing any particular sediment. 



In order to understand the history of certain sedimentary rocks, it is 

 necessary to know the causes, in many instances of physico-chemical na- 

 ture, that have produced changes in the sediments. Among such changes 

 are silicification, phosphatization, and dolomitization. But a complete 

 knowledge of the processes whereby such changes were brought about will 

 not suffice, because the origin of the unchanged sediment needs to be 

 explained, and it may have undergone other changes than one of those 

 mentioned. 



Other instances of valuable chemical and physico-chemical researches 

 might Ije given and the reason of their inadequacy for the interpretation 

 of sediments indicated, but those mentioned seem sufficient to illustrate 

 the point in mind. 



