Wallace, in his 'Island Life,' by making use of Professor A. Geikie's results as to the rate of denudation of matter by rivers from the area of their basins, and estimating the average rate of deposition, concludes that 'the time required to produce this thickness of rock [Professor Haughton's maximum of 177,000 feet] at the present rate of denudation and deposition is only 28,000,000 years.' Our lower assumption of 130,000 feet thickness would give only 20,000,000 years--a rate of 1 foot in 154 years. Again, if we prefer round numbers to start with, we have only to assume that the age of the whole Tertiary period, with its 3,000 feet thickness, is 3,000,000 years (_i.e._, 1,000 feet in 1,000,000 years, or 1 foot in 1,000 years, surely an excessively slow rate); then 130,000,000 years would bring us to the bottom of the Laurentian or pre-Cambrian deposits. Of course, it is a pure assumption that the same rate of destruction and sedimentation applies to the whole of the strata; but we know nothing to the contrary, especially if we consider the average periods, the quick periods of extra activity, taken with the slow periods or those of standstill. Dana estimated the length of the whole Tertiary period at one-fifteenth of the Mesozoic and Palæozoic combined. If we take the duration of the Tertiary period, as before, as 3,000,000 to 4,000,000 years, the total will amount to from 45,000,000 to 60,000,000 years. Lastly, Walcott[28] has estimated the duration of the Palæozoic, Mesozoic, and Cænozoic or Tertiary epochs at about 17,000,000, 7,000,000 and 3,000,000 years respectively, giving 27,700,000 years from the beginning of the Cambrian; and Williams[29] has calculated the relative duration of the smaller epochs. See the table on p. 149. The results of all these calculations fall surprisingly well within the limits of Lord Kelvin's allowance. Of course they are based upon assumptions, but none of them is inherently unreasonable; and it was my purpose to draw attention to the surprising coincidence in the closeness of these results, perhaps too good to be true. Such calculations are considered close enough if they range within a few multiples of each other. [28] 'Geological Time as indicated by the Sedimentary Rocks of North America.' _Proc. Amer. Assoc. Adv. Sci._, xlii., 1893, pp. 129-169. [29] Henry Shaler Williams, 'Geological Biology.' New York, 1895.