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ELEMENTS OF THE GEOLOGICAL TIME-SCALE.
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accuracy may be attained by further investigation. It is doubtful if it is possible with our present knowledge to reach an estimate in years or centuries, of the actual length of geological time, which is within 100 or perhaps 200 per cent. of the truth. We may accept Dana's estimate of at least 48,000,000 of years, or Geikie's of from 100,000,000 to 680,000,000. We find at one extreme the ancient theory of 6,000 years 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 river 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 sedimentary 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 estimates 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 abnormal 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 importance 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

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