_How the method has been applied._—Having determined what appears to be the probable average rate of subaërial denudation, we may now proceed to consider the way in which this rate has been applied to measure past geological time. There are two ways in which it may be applied for this purpose. It may (1) be applied directly: knowing the thickness of strata which may have been removed by denudation, we can easily tell, from that rate, the time it required to effect their removal. If we have evidence, for example, that at some epoch 1,000 feet of stratified rock were carried away, then, on the assumption that the rate of denudation was the same at that epoch as now, we have 1,000 × 6,000 = 6,000,000 years as the required time. (2) It may be applied indirectly: knowing the thickness of the strata, we may estimate the time required for their formation. This is the way in which it has usually been applied, but, as we shall see, it is the less satisfactory way of the two. Dr. A. Geikie gives the land area of the globe as 52,000,000 square miles, and that of water as 144,712,000 square miles.[30] We may thus take the proportion of land to water roughly as 1 to 3; about one-quarter of the earth’s surface being land, and three-quarters water. One foot, therefore, removed off the surface of the land would cover the whole globe with a layer 3 inches thick, or the entire sea-bottom with a layer 4 inches thick. [Footnote 30: _Physical Geography_, p. 103.] If we knew the total quantity of stratified rock on the globe, we could easily tell the time that would be required for its formation. Most geologists would, I believe, be inclined to admit that, if spread uniformly over the entire globe, it would form a layer of at least 1,000 feet in thickness. In such a case the time required for its deposition would be as follows: 1,000 × 6,000 × 4 = 24,000,000 years.