SHAW: SEDIMENTATION 517 



ent-day sediments and sedimentary processes and that as great 

 a diversity of phenomena as possible should be investigated." 

 In the report of the Petrologists' Club committee on sedimentary 

 rocks ^ it is stated that "The main object of petrologic descrip- 

 tions of sedimentary rocks is evidently the interpretation of 

 sedimentary records, though such descriptions should be useful 

 also for identification, classification, economic purposes, and 

 miscellaneous reference." 



The need of carefully recorded descriptions of the physical 

 characteristics of ancient sediments is especially worthy of em- 

 phasis. We have as yet no adequate color scale and statements 

 concerning texture are usually only the crudest approximations. 

 In geologic field work the proportions of various sizes of grains 

 are rarely determined. The term "porosity" is used promiscu- 

 ously for "total volume of pores," for "size of pores" and for 

 ' 'perviousness ' ' — entirely distinct concepts . Mineralogic or chem- 

 ical constitution is rarely determined in detail ;^ — our classic table 

 of average chemical composition of sedimentary rocks is, I be- 

 lieve, defective;^ and shape of grains in noted onl^ occasionally. 



Methods of collecting specimens are much in need of stand- 

 ardization. Many samples represent a considerable thickness 

 of strata — from a few inches to a foot or more, — within which 

 more or less difference in conditions of deposition may occur. 

 The resulting complexity in the sample greatly reduces the value 

 of any mechanical analysis. 



Though already recognized, the fact is still worth pondering 

 that the bulk of our knowledge of sedimentation has come more 

 or less inductively from the study of sedimentary rocks, condi- 

 tions of deposition being inferred from general principles of phys- 

 ics and chemistry. Studies of present-day agents, processes, and 

 kinds of deposits would furnish a much better foundation, though 

 admittedly such studies would not be complete, because the 

 conditions of deposition of most strata, particularly as to areal 

 extent and also in other respects, are not fully represented any- 

 where at present. 



' Mimeographed report signed by M. I. Goldman, D. F. Hbwett, G, S. Rogers, 

 and E. W. Shaw. 



* Shaw, E. W., Sulphur in rocks andin river waters. This Journal 5: 484. 1915. 



