272 Joseph Barrell. 



in sedimentation, and shows that sedimentation is "not 

 a continuous process, even during a stage of crustal 

 depression." Therefore the stratigraphic record is 

 replete with "breaks" of varying time lengths, the 

 non-seeable greater disconformities and the lesser but 

 more numerous diastems. 



Part III treats of the estimates of time based on geo- 

 logic processes, on erosion, sedimentation, hypothesis 

 of compound rhythms, amount of oceanic salt, and on 

 the loss of primal heat. In this presentation we get a 

 more adequate idea of the quantitative lengthening of 

 geologic time. "Measurements of time based on radio- 

 activity" is the subject of Part IV, which is a worthy 

 associate of A. Holmes's "Radioactivity and the Meas- 

 urement of Geological Time," published in 1915. Fin- 

 ally, in Part VI, ' ' Convergence of Evidence on Greologic 

 Time and its Bearings," the geological and physical 

 arguments are bound together into a unity, resulting in 

 the conclusion that at least 550 million, and a maximum 

 of 700 million years have elapsed since the beginning 

 of the Cambrian. This is, moreover, less than one-half 

 of geologic time, for the Laurentian or post-Ladogian 

 granites, the oldest great invasions of igneous magmas 

 into vast thicknesses of sedimentary formations, have 

 an age as great as 1,400 million years. The earth is, 

 therefore, much older than even this great figure. 



Harlow Shapley, the astronomer, accepts Barrell 's 

 geologic time estimates, and says: "We may study the 

 stars, indeed, with the aid of fossils in terrestrial rocks, 

 and acquire knowledge of atomic structure from the 

 climates of Precambrian times." "In the growth of 

 our concepts of the age of the earth, Barrell's discus- 

 sion is likely to mark an epoch because of its consistent 

 carefulness, its great expansion of geologic time beyond 

 the commonly accepted limits, and its decided rebellion 

 against the stringent limitations set by Kelvin and later 

 physicists." 



Isostasy. 



It is interesting to note Barrell's early independence 

 of mind and his love for quantitative studies and philo- 

 sophical analyses in geology, as shown in his notice in 

 this Journal in 1904 of T. M. Reade's "Evolution of 

 Earth Structure." In August, 1906, he notices, also in 



