812 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



"probable length of Glacial and post-Glacial time together is 30,000 or 40.000 

 years, more or less ; but an equal or considerably longer preceding time, while 

 the areas that became covered by ice were being uplifted to high altitudes, may 

 perhaps with good reason be also included in the Quaternary era, which then 

 would comprise some 100,000 years." *^ 



On the other hand, Chamberlin and others, recognizing the com- 

 plexity of the ice-advances and basing their estimates npon the degree 

 of erosion of the older glacial deposits, obtain a period of 300,000 to 

 1,020,000 years from the present to the Kansan glacial stage, and an 

 unknown greater time to the beginning of the Pleistocene.*^ McGee, 

 comparing the relative erosion of river valleys in post-Glacial and post- 

 Columbian (early or middle Pleistocene) time and the work of erosion 

 in the Tertiar}^, obtains a mean estimate of 200,000 years as the duration 

 of post-Columbian time and 90,000,000 years for the Cenozoic.*^ 



Groodchild takes the time since the Iclose of the Glacial period as 20,000 

 years. He then makes an estimate of the time needed for the erosion of 

 the Skye volcano. This he takes as beginning to erupt some time in the 

 Oligocene, completed by the close of the Miocene, and as having an initial 

 height of 8,000 feet. Choosing an erosion rate for its gabbroic rocks as 

 one foot in 2,000 years gives an estimate of 16,000,000 years since the 

 close of the Miocene.^* 



Dana obtained ratios of the relative durations of the geologic periods 

 by comparing the volumes of sediment deposited in them. Taking the 

 Quaternary as 1,000,000 years, the length of the Tertiary is 3,000,000. 

 Ward, in the Fifth Annual Eeport of the United States Geological 

 Survey, gives a time ratio to the Quaternary-Eecent as 1, the Miocene- 

 Pliocene as 1, and the Eocene as 1, and Williams adopts these ratios.*^ 



These estimates by American geologists are no doubt the basis for a 

 very common statement of the length of the Tertiary as 3,000,000 years. 

 They were probably based originally on a comparison of the rather thin 

 Tertiary deposits of the Atlantic coastal plain, the presence of many 

 unconformities being then unappreciated. At that time the magnitude 

 of Tertiary erosion and sedimentation in the regions of mountain-making 

 was also uncomprehended. Year by year it becomes iclearer that, since 

 the opening of the Miocene, uplifts resulting in a greater or less com- 

 pletion of erosion cycles have been repeated several times. Blackwelder 



*iUpham: Am. Jour. Sci., vol. xlv, 1893, pp. 217, 218. 



42 Chamberlin and Salisbury : Geology, vol. iii, 1906, p. 420. 



*3 w J McGee : Note on the age of the earth. Science, vol. xxi, 1893, p. 309. 



** .T. G. Goodchild : Vice-presidential address. Royal Physical Society of Edinburgh. 

 Nov.. 1896, pp. 263-265, 1897. 



^ H. S. Williams : The elements of the geological time scale. Jour. Geol., vol. i, 1893, 

 p. 295. 



