NO. G AGE OF THE EARTH —BECKER 15 



geograph}- ' it appears that the mean' area of the continent of North 

 America has been about 8/10 of its present area. Wliile the paleo- 

 geography of no otlier continent seems to have been so satisfactorily 

 treated, it is generally acknowledged that the continents have grown more 

 stable and the ocean deeper. So far as existing continents are concerned, 

 therefore, it is fair to conclude that the mean land exposure has been 

 somewhat less than the actual land area. Many paleontologists, however, 

 think themselves driven by zoological reasoning to believe in the former 

 existence of extensive continents bridging the great oceanic basins of the 

 present day. This seems to me mechanically improbable and some great 

 authorities such as Sir Archibald Geikie ^ refuse to concede the point. If 

 the existence of such sunken continents of unknoAvn extent were to be 

 assumed, there would be no such thing as approximating to a mean land 

 area. All I can do, therefore, is to point out that the effect of such 

 bodies of land would be greatly to reduce the appropriate estimate of the 

 earth's age by the method under discussion. 



Assuming the permanence of continental areas, the uncertainty of the 

 mean area is partially offset by marine denudation ; for when shallow seas 

 invaded the continents, breakers and the tidal wave must have triturated 

 far more rock than is ground up on the shores of present continents. 



A careful study of the areas of exposure of the principal geological 

 formations was made by the late distinguished physical geographer, 

 Lieutenant-General Alexis von Tillo. This includes the Archean and the 

 younger eruptives, the results being expressed in hundredths of the total 

 surveved area. The following is an extract from von Tillo's table:'' 



Continent. Archean. Eruptives. Total. 



Europe 20.6 1.3 21.9 



Asia 17.7 4.7 22.4 



Africa 18.4 2.2 20.6 



Oceanica 20.0 4.8 .24.8 



North America 27.2 5.5 32.7 



South America 18.7 4.C 23.3 



Mean 20.3 4.0 24.3 



The most recent geological map of ISTorth America (compiled by Mr. 

 Bailey Willis) shows that the relative area of exposed feldspathic rocks 

 on this continent is not so large as was supposed when von Tillo wrote, 

 and, though I have made no minute measurements, this exposure as now 

 mapped seems not to exceed 25 per cent. With this emendation, von 

 Tillo's table shows a truly remarkable uniformity throughout the world, all 

 the figures lying between a fourth and fifth of the total' area, the average 



iBull. Geol. Soc. Am., vol. 20, 1910, p. 427. 



2 Text-book of Geology, 4th ed., lOO."?, p. 1365. 



3 Comptes Rendus, Paris, vol. 114, 1892, pp. 246, 967. 



