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. [Footnote 33: _Quart. Journ. of Science_, July 1877; _Climate and Cosmology_, chap. xvii.] 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.