42 PROCEEDINGS OF THE ANN ARBOR MEETING 



q"uotations must suffice. The work should be studied by all interested in 

 the fundamentals of geology. 



Barren's studies of rhythms affecting sedimentation have been indi- 

 cated in the discussions of his researches on the relief factor and climate 

 factor in deposition. The first paragraph in his "Ehythms and the meas- 

 urements of geologic time"' is as follows : 



"Nature vibrates with rliythms, climatic and diastrophic, tliose finding 

 stratigrapliic expression ranging in period from the rapid oscillation of sur- 

 face waters, recorded in ripple-marks, to those long-deferred stirrings of the 

 deep imprisoned titans which have divided earth history into periods and eras. 

 The flight of time is measured by the weaving of compositive rhythms — day 

 and night, calm and storm, summer and winter, birth and death — such as these 

 are sensed in the brief life of man. But the career of the earth recedes into 

 a remoteness against which these letter cycles are as unavailing for the meas- 

 urement of that abyss of time as would be for the measurement of human 

 history the beating of an insect's wing. We must seek out, then, the nature 

 of those longer rhythms whose very existence was unknown until man by the 

 light of science sought to understand the earth. The larger of these must be 

 measured in terms of the smaller, and the smaller must be measured in terms 

 of years. Sedimentation is controlled by them, and the stratigraphic series 

 constitutes a record, written on tablets of stone, of these lesser and greater 

 waves of change which have pulsed through geologic time." 



The paper is divided into six parts, as follows : I, Rhythms in denuda- 

 tion ; II, Ehythms in sedimentation : III, Estimates of time based on 

 geologic processes; IV, Measurements of time based on radioactivity; V, 

 The age of the Llano series, Texas; YI, Convergence of evidence on geo- 

 logic time and its bearings. 



In Part I it is shown that, because of the present relatively high stand 

 of the continents, the rate of denudation is probably far in excess of the 

 average for geologic time — "the present mean rate may be twice the mean 

 for the whole of the Cenozoic and 10 or 15 times the rate for all of o-eo- 

 logic time since the opening of the Paleozoic.'^ ^^ From this it follows 

 *^that time is far longer than those estimates which have been based on a 

 hypothesis that the present rate is a mean which applies to the geologic 

 past." ^^ 



In Part II particular emphasis is put on breaks in the continuity of 

 deposition. The word disconformity is applied according to current 

 usage, but for cessations of minor magnitude in deposition the term 

 diastem is proposed. The admission of the great importance of lost in- 

 tervals in deposition destroys the basis "for estimating tlie time of the 



** Op. cit.. I). TTG. 



