A writer in the Gentleman’s Magazine[74] (Professor Huxley?), whose article I am only able to quote at second-hand, makes an estimate which, though much lower than the above, is still of enormous magnitude, as follows:—
| Feet. | Years. | |||
| Laurentian | 30,000 | 30,000,000 | ||
| Cambrian | 25,000 | 25,000,000 | ||
| Silurian | 6,000 | 6,000,000 | ||
| Old Red and Devonian | 10,000 | 10,000,000 | ||
| Carboniferous | 12,000 | 12,000,000 | ||
| Secondary | 10,000 | 10,000,000 | ||
| Tertiary and Post Tertiary | 1,000 | 1,000,000 | ||
| Gaps and unrepresented strata | 6,000 | 6,000,000 | ||
| Total | 100,000,000 |
Mr. Darwin, arguing upon Sir W. Thompson’s estimate of a minimum of ninety-eight and maximum of two hundred millions of years since the consolidation of the crust, and on Mr. Croll’s estimate of sixty millions, as the time elapsed since the Cambrian period, considers that the latter is quite insufficient to permit of the many and great mutations of life which have certainly occurred since then. He judges from the small amount of organic change since the commencement of the glacial epoch, and adds that the previous one hundred and forty million years can hardly be considered as sufficient for the development of the varied forms of life which certainly existed towards the close of the Cambrian period.
On the other hand, Mr. Croll considers that it is utterly impossible that the existing order of things, as regards our globe, can date so far back as anything like five hundred millions of years, and, starting with referring the commencement of the Glacial epoch to two hundred and fifty thousand years ago, allows fifteen millions since the beginning of the Eocene period, and sixty millions of years in all since the beginning of the Cambrian period. He bases his arguments on the limit to the age of the sun’s heat as detailed by Sir William Thompson.
Sir Charles Lyell and Professor Haughton respectively estimated the expiration of time from the commencement of the Cambrian at two hundred and forty and two hundred millions of years, basing their calculations on the rate of modification of the species of mollusca, in the one case, and on the rate of formation of rocks and their maximum thickness, in the other.
This, moreover, is irrespective of the vast periods during which life must have existed, which on the development theory necessarily preceded the Cambrian, and, according to Mr. Darwin, should not be less than in the proportion of five to two.
In fine, one school of geologists and zoologists demand the maximum periods quoted above, to account for the amount of sedimentary deposit, and the specific developments which have occurred; the other considers the periods claimed as requisite for these actions to be unnecessary, and to be in excess of the limits which, according to their views, the physical elements of the case permit.
Mr. Wallace, in reviewing the question, dwells on the probability of the rate of geological changes having been greater in very remote times than it is at present, and thus opens a way to the reconciliation of the opposing views so far as one half the question is concerned.
Having thus adverted to the principles upon which various theorists have in part based their attacks on the problem of the estimation of the duration of geological ages, I may now make a few more detailed observations upon those later periods during which man is, now, generally admitted to have existed, and refer lightly to the earlier times which some, but not all, geologists consider to have furnished evidences of his presence.
I omit discussing the doubtful assertions of the extreme antiquity of man, which come to us from American observers, such as are based on supposed footprints in rocks of secondary age, figured in a semi-scientific and exceedingly valuable popular journal. There are other theories which I omit, both because they need further confirmation by scientific investigators, and because they deal with periods so remote as to be totally devoid of significance for the argument of this work.