750 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



Looking at the geological side of the problem from the standpoint oi' 

 the control of sedimentation by baselevel^ a snbject which had at that 

 time recently engaged the writer^s attention, rather than by the present 

 rate of denudation/ he stated in 1906 to Professor Boltwood, the first 

 to seriously develop the evidences of age based on radioactive processes, 

 that no real conflict appeared to exist between the geological facts and 

 the new physical evidence. The iconflict was only with one of the several 

 possible interpretations of that evidence, though one which was generally 

 accepted. 



This article, as previously stated, is primarily an analysis of the geo- 

 logic evidence, taking in especially the influence of the fundamental 

 factor of composite rhythms, but it wouhi be incomplete if it discussed 

 only the interpretation of sedimentary procesvses in terms of rhythms. 

 Such a discussion might fail to carry conviction of the great lengthening 

 which it implies in the vista of geologic time, and at best would be 

 qualitative rather than quantitative. Consequently, in Part III, a revie^^- 

 is given of the methods, of measurement based, first, on erosion and 

 sedimentation; second, on chemical denudation and the sodium in the 

 sea; and third, on the thermal gradient of the crust. In tliis review 

 care is taken to examine the postulates which underlie these methods 

 and which have resulted in current estimates of geologic time very much 

 smaller than the estimates reached by the consideration of sedimentary 

 rhythms and the measurements based on radioactive ])rocesses. 



Another method is that of the detection of rhythms in parts of the 

 sedimentary series, and the correlation of these rhythms with known 

 climatic (cycles. The use of the precession cycle has given very long 

 estimates for these small pieces of geologic time. There are suggestions, 

 however, derived from postglacial time, that the rhythm which is 

 prominent is not so long as the precession cycle of 21,000 years, but is 

 probably a rhythm iji solar energy several thousand years in length. 

 Such a conclusion comes into ajccord with the other lines of analysis of 

 this paper, extending geologic time far l^eyond the generally accepted 

 maximum limit of a hundred million years. 



This is followed in Part IV by a presentation of the evidence from 

 ladioactivity, as based on the accumulation of helium and lead in radio- 

 active minerals. Such a presentation seems all the more necessary, 

 since no adequate treatment of tliat subject has been published in Amer- 

 ican geological -literature. It is made easier at the present time by the 



2 Joseph Barren : Relative geological importance of continental, littoral, and marine 

 sedimentation. .Tonrnal of Geology, vol. xiv, part 1, 1906, pp. .316-856. 



