This would not, however, represent the age of the stratified rocks. It would only represent the time required to deposit the rocks which we have assumed to be now in existence. The greater mass of sedimentary rocks has been formed out of previously existing sedimentary rocks, and these again out of sedimentary rocks still older. The oldest known sedimentary rocks are the Laurentian; but these are believed by geologists to have been formed from still older sedimentary rocks. It is therefore evident that the materials composing our stratified beds must have passed through many cycles of destruction and re-formation. The materials of some of the recent formations, for example, may have passed through denudation and deposition a dozen of times over.[[31]] The time required to have deposited at a given rate the present existing mass of sedimentary rocks is probably but a small fraction of the time required to have deposited at the same rate the total mass that has actually been formed. Few geologists, I think, who will duly reflect on the subject, will deem it too much to say that the present existing stratified rocks have on an average passed at least thrice through the cycle of destruction and re-formation. If this be admitted, then the 1,000 feet of stratified rock represent, not a period of 24,000,000 years, but a period three times as great, viz. 72,000,000 years.
It is impossible to tell from geological data the actual age of the stratified rocks; but this is not required. What we require is, as already stated, not their actual age, but an inferior limit to that age.
Method as applied by Professor Haughton.—Professor Haughton estimates the mass of the stratified rocks down to the time of the Miocene Tertiary period as being 177,200 feet in thickness, and covering an area equal to that of the sea. The present rate of subaërial denudation he considers to be equal to one foot removed off the surface of the land in 3,090 years. If the proportion of land to water be taken as 52 to 145, it thus requires 8,616 years to deposit one foot of sediment over the bed of the ocean, and consequently this is the rate at which strata are at present being formed. This would give 8,616 × 177,200 = 1,526,750,000 years for the age of the stratified rocks. But he assumes the rate of denudation to have been ten times greater in geological time than at present. This consequently reduces the age of the rocks to 152,675,000 years. By adding one-third for the time which has elapsed since the Miocene Tertiary period he gets 200,000,000 years as a minimum length of geological time.[[32]]
The validity of this result rests upon what appear to me to be two very doubtful assumptions. It is assumed in his calculations that the total amount of strata formed during past ages (not the amount presently remaining) was equal to a mass 177,200 feet in thickness, covering the entire area of the ocean. This is certainly doubtful. It may have been as great, for anything that can be proved to the contrary; but we have no evidence that it was so. Certainly there is no evidence that the rate of subaërial denudation during past ages was ever ten times as great as it is now. But how is a length of 200,000,000 years to be reconciled with the age of the sun’s heat? The stratified rocks may be as old as this, but assuredly they are not if gravitation was the only source from which the sun derived his energy.
Method as applied by Mr. Alfred R. Wallace.—Mr. Wallace adopts Professor Haughton’s estimate of 177,200 feet for the maximum thickness of the sedimentary rocks. But, instead of supposing, like Professor Haughton, the products of denudation to be uniformly spread over the entire sea-bottom, he supposes them spread over a belt of merely 30 miles broad, extending along the entire coast-line of the globe, which he estimates at 100,000 miles. This gives an area of 3,000,000 square miles on which the denuded matter of the whole land area of 57,000,000 square miles is deposited. These two areas are to one another as 1 to 19, and thus it follows that deposition goes on 19 times as fast as denudation. The rate of denudation he takes as one foot removed off the surface of the land in 3,000 years, so that the rate of deposition would be about one foot in 158 years, and consequently the time required to deposit the 177,200 feet of rock would be
177,200 × 158 = 27,997,600 years.
This is a period double what the gravitation theory of the source of the sun’s energy can afford. And if the rate of denudation be taken at one foot in 6,000 years, which is, as we have seen, probably nearer the truth, then this would make the age of the stratified rocks 56,000,000 years.
There seems to be a little ambiguity about Mr. Wallace’s result. Do the 177,200 feet represent the quantity of rock which presently exists, or do they represent the total quantity which has been formed during all past ages? If the former, then the 28,000,000 years are but a fraction of the time which must have been required; for, as we have been shown, the materials composing the stratified rocks have, on an average, been deposited at least three or four times over. If, on the other hand, the thickness is meant to represent the total quantity of rock which has been formed during the whole of past geological time, then the question arises, by what means could this quantity possibly be ascertained? In other words, how was the relation between the present quantity and the total quantity ascertained? But in either case the result is wholly irreconcilable with the gravitation theory of the source of the sun’s heat.
Method as applied directly.—We have seen that it is impossible to determine the actual age of the earth from the stratified rocks, even if we knew with perfect accuracy their present total amount. We have also seen that from the rate of deposition we cannot fix with any degree of certainty a minimum value for the age of these rocks. We can, however, by means of the first or direct application of the method, assign with tolerable accuracy, as was shown on a former occasion,[[33]] a minimum age to the earth. We can be far more certain of the time which must have been required to remove by denudation, say, a thousand feet of rock than we can possibly be of the time required to have deposited a thousand feet of sediment. The thousand feet of sediment may, under certain conditions, have been deposited in a hundred years, while under other conditions they may have required a million of years. In fact, nothing can be more uncertain than the rate of deposition: it depends upon such a multitude of circumstances. At the mouth of a great river, for example, a foot of sediment may be deposited in a single day, whereas in some places, as in mid-ocean, it may require a million of years to deposit the same amount. But in reference to subaërial denudation no such uncertainty exists.
The utter inadequacy of a period of 20,000,000 years for the age of our earth is demonstrable from the enormous thickness of rock which is known to have been removed off certain areas by denudation. I shall now briefly refer to a few of the many facts which might be adduced on this point.