ELEMENTS OF THE GEOLOGICAL TIME- SCALE. 293 



accuracy may be attained by further investigation. It is doubt- 

 ful if it is possible with our present knowledge to reach an esti- 

 mate in years or centuries, of the actual length of geological 

 time, which is within lOO or perhaps 200 per cent, of the truth. 

 We may accept Dana's estimate of at least 48,000,000 of years, 

 or Giekie's of from 100,000,000 to 680,000,000. We find at one 

 extreme the ancient theory of 6,000 yeairs and at the other 

 McGee's possible maximum of 7,000,000,000 years. 



The rate of accumulation of sediment over the bottom of the 

 sea may vary between the limits of one foot in 730 years and one 

 foot in 6,800 years, as pointed out by Geikie, the figures being 

 based upon the estimated proportion between the annual discharge 

 of sediment in cubic feet and the area of river basins in square 

 miles, in the case of the rivers Po and Danube. The estimate of 

 680,000,000 of years, quoted above, is dependent upon the 

 assumption that the total thickness (maximum) for the sediment- 

 ary deposits is not less than 100,000 feet, and that the average 

 rate of accumulation was not more rapid than that now going on 

 at the mouth of the Danube, based upon Bischof's determination 

 of the amount of sediment and matter in solution in the Danube 

 at Vienna. It may be a query worth considering whether the esti- 

 mates based upon the examination of the amount of suspended 

 and dissolved matter in river water are not likely to err in the 

 direction of too small amount of matter by reason of the abnor- 

 mal precipitation along the course of the river incident to the 

 presence of salts and acids put into the river by man. If the 

 rate of the river Po were taken the length of time would be 

 73,000,000 of years instead of 680,000,000. 



The actual length of time in years, however, is of less import- 

 ance to the geologist than the relative length of time for each of 

 the ages, and these latter, the time -ratios of Dana, are clearly 

 deducible from the physical thickness and size of constituent 

 particles of sedimentary rocks. Relative thickness is certainly 

 one of the elements in the determination of the time values of 

 the geological formation, and the fields for investigation along 

 which greater accuracy is to be reached cover the problems of 



