32 PKOCKKDINtJS OF THE ANN ARBOR MEETING 



Barrell, after recognizing that the characteristics of certain important 

 geologic formations were not compatible witli Avhat was known of the 

 features of niariiu' sediments, undertook a comprehensive study of the 

 characteristics of continental and littoral deposits, the only other classes 

 of deposits to wliicli tliey could belong. He worked mostly by compila- 

 tion from literature and l)y deduction from well established principles, 

 but he made a few simple experiments on the formation of mud-cracks. 

 In many respects liis work was not so detailed as desirable. For instance, 

 he gave no mechanical analyses of the sediments studied, sorting is dis- 

 cussed in only a general Avay, and more attention might profitably have 

 been paid to the mineralogic and chemical composition of sediments. 

 He himself recognized and mentioned these deficiencies, thereby offering 

 guidance to furtlier studies in many of the important problems in sedi- 

 mentation. 



It is practicable in a brief review, such as this necessarily is, merely to 

 indicate some of tlie important results of Barrell's studies, and it should 

 be stated in fairness to otliers that many others had prior to Barrell done 

 more or less work along similar lines : but to him belongs the credit of 

 assembling scattered observations and conclusions, organizing them into 

 a consistent body of usable criteria, and applying these criteria to the 

 interpretation of many formations. Certain factors in the control of 

 sedimentation and certain distinctive features of some kinds of sediment? 

 will be briefly discussed, the remarks being based on his principal articles. 

 Reviews l)y liim and liis discussions of papers l)y others are not consid- 

 ered, because to do so would unluly extend tlie length of this statement. 



77//; in:Lii:r factor jx deposition 



A concept of funchmuMital importan(!e in the consideration of problems 

 of sedimentation is that of baselevel. That there is a terrestrial base- 

 level, the goal toward which all erosional agencies on the land are work- 

 ing,, and that there is a marine baselevel, which is effective wave base, 

 toward which marine agencies are working, have long been recognized, 

 but they had not been sufficiently considered in their relations one to the 

 other. Barrell says : 



••Thus sediments wliose interpretation from [sicj the basis of earth history 

 liave been eharaeteristically dejiosited with respect to a nearly horizontal con- 

 trolling surface. This surface of control is baselevel, but for continents and 

 marine deposits the baselevel is determined by different agencies and is a 

 word of more inclusive content than the sense in which it has generally been 

 used by physiographers as a level limiting the depth of fluviatile erosion. 

 Sedimentation as well as erosion is controlled by baselevel. and baselevel. local 



