WORK OF JOSEPH BARRELL OX SEDIMENTATION 43 



accumulation of the whole formation by means of an assmuecl rate of 

 continuous denudation and corresponding sedimentation." ^^ 



In Part III several estimates of geologic time are critically reviewed 

 with reference to "the relations of methods to results and the assumptions 

 on which those methods rest/' Estimates by Lyell, Croll, Kelvin, Sollas, 

 Walcott, Goodchild, Becker, and others are discussed. A fundamental 

 error in many of these estimates lies in the assumption of continuous 

 erosion and continuous deposition at present rates. This assumption 

 invalidates the age estimates by Becker based on the present sodium 

 chloride content of the ocean. These estimates are, therefore, too low. 

 Goodchild's estimate of 704,000,000 years since the beginning of the 

 Cambrian is considered of the proper order of magnitude. Becker's esti- 

 mate of the age of the earth from a combination of data based upon the 

 supposed "curve of fusion of diabase with respect to pressure," the tem- 

 perature gradient of the earth according to increase in depth, data on 

 isostasy, and data on radioactive minerals is severely criticized in these 

 words : "Thus, assumption is built on assumption into a many-storied 

 structure and the whole rests on a foundation of quicksand." *^ 



Part IV consists of a careful review of the subject of radioactive min- 

 erals in their bearing on the age of the earth. Accumulation of helium, 

 pleochroic halos, and accumulation of lead are discussed in considerable 

 detail. Barrell's data are taken from Eutheford, Ellen Gleditsch, Arthur 

 Holmes, Eamsay and Soddy, Strutt, Boltwood, and others. The helium 

 ratio indicates an age of 623 to 715 million years as the age of the Lower 

 Precambrian of Renfrew County, Ontario, Canada; but all that this ratio 

 tells is that "the age of a mineral is greater than a certain minimum 

 value." *^ The lead ratio indicates an age for the larger helium ratio 

 above given as 1,500 million years. 



In Part V it is shown that Becker's conclusions on the uranium min- 

 erals from the pegmatite dikes of the Llano series, Texas, are unjustified, 

 'and that after eliminating certain analyses for specific reasons, the others 

 serve "as a reliable means of measuring the age of the Llano series and 

 add their weight to the value of the method." *^ 



In Part VI the different lines of evidence are brought together and on 

 pages 884 and 885 a new table of geologic time is presented. The begin- 

 ning of Cambrian time is given as between 550 and 700 million years 

 ago; Mesozoic, between 135 and 180 million years ago; Cenozoic, between 



39 Op. cit, p. 798. 



^«0p. cit., p. 841. 



" Op. cit., p. 847. 



"- Op. cit., p. 871. 



