HOW OLD IS TBI: EARTH? 



157 



about forty- eight million years since the formation of the oldest 

 fossilif erous rocks ; and by Alfred Russel Wallace, who concludes 

 that this time has probably been only about twenty-eight million 

 years. With these, rather than with the foregoing, we may also 

 place Mr. T. Mellard Reade's recent estimate of ninety-five million 

 years, similarly derived. Again, Mr. C. D. Walcott, in his vice- 

 presidential address before Section E of the American Associa- 

 tion for the Advancement of Science, in its meeting last August, 

 gave his opinion, from a study of the sedimentary rocks of the 

 western Cordilleran area of the United States, that the duration 

 of time since the Archaean era has been probably some forty-five 

 million years. 



Selecting the lowest of these various estimates as the nearest 

 in accord with the conclusions of physical and astronomical 

 science, let us scrutinize the processes of Wallace's measurements 

 and computations. It has been found that the rates at which 

 rivers are lowering the altitudes of their basins by the transpor- 

 tation of sediments to the sea vary from an average of one foot 

 taken from the land surface of its hydrographic basin by the 

 river Po in seven hundred and thirty years, to one foot by the 

 Danube in six thousand eight hundred years. As a mean for all 

 the rivers of the world, Wallace assumes that the erosion from 

 all the land surface is one foot in three thousand years. The sedi- 

 ments are laid down in the sea on an average within thirty miles 

 from the coast, and all the coast lines of the earth have a total 

 measured length, according to Dr. James Croll and Mr. Wallace, 

 of about one hundred thousand miles, so that the deposition is 

 almost wholly confined to an area of about three million square 

 miles. This area is one nineteenth as large as the earth's total 

 land area ; hence it will receive sediment nineteen times as fast as 

 the land is denuded, or at the rate of about nineteen feet of strati- 

 fied beds in three thousand years, which would give one foot in 

 one hundred and fifty-eight years. With this Wallace compares 

 the total maxima of all the sedimentary rocks of the series of 

 geologic epochs, measured in whatever part of the earth they are 

 found to have their greatest development. Prof. Haughton esti- 

 mates their aggregate to be one hundred and seventy-seven thou- 

 sand two hundred feet, which, multiplied by one hundred and 

 fifty-eight, gives approximately twenty-eight million years as the 

 time required for the deposition of the rock strata in the various 

 districts where they are thickest and have most fully escaped 

 erosion and redeposition. 



Most readers, following this argument, would infer that it 

 must give too large rather than too scanty an estimate of geologic 

 duration; but to many students of the earth's stratigraphy it 

 seems more probably deficient than excessive. All must confess 



