Unfortunately, we possess no data whatever for calculating the age of the successive geological strata. Thanks to Lyell, the theory of violent universal cataclysms has been done away with. It is more probable that the same agencies have acted which are now changing the aspect of the globe; and these changes are slow, as far as we know them--at least, as far as the formation of sedimentary strata is concerned, and these alone we have to deal with. Various calculations have been made, based upon the denudation of the mountains, the filling up of the valleys by the débris, the formation of deltas, etc. The results give enormous stretches of time, but all of them unsatisfactory, because the methods are so very local in their application. The least objectionable attempt is that which, based upon astronomical calculations, tried to fix the height of the last Glacial epoch[26] at about 200,000 years ago, and asserted that since its beginning in the Pliocene epoch as many as 270,000 years have elapsed. The duration of the whole Tertiary period has by the same authorities been fixed approximately at 3,000,000 to 4,000,000 years. Beyond this we cannot venture without the wildest speculation; but we know to a certain extent the thickness of the various sedimentary strata, which amount in all to from 100,000 to 175,000 feet--on the average perhaps 130,000 feet, or about twenty miles. [26] James Croll: 'On Geological Time, and the Probable Date of the Glacial and Upper Miocene Period,' _Philos. Magazine_, xxxv., 1868, pp. 363-384; xxxvi., pp. 141-154; 362-386. Unless we prefer giving up all attempt at calculation as absolutely hopeless, and thus resign the whole problem, we must at least try to arrive at some results, and then see if these cannot reasonably be made use of. Neither geologist nor physicist, and no zoologist, would accept the suggestion that these 130,000 feet of stratified rocks have been deposited within only as many years, although the average rate of deposit would in that case be not more than 1 foot per year. On the other hand, an indignant protest is raised against the assumption of 1,000,000,000 years. Lord Kelvin[27] has come to the conclusion (from data which various other authorities regard as very unsatisfactory) that not much more than 100,000,000 years can have elapsed since the molten globe acquired a consolidated crust. Further time must have passed before the surface had become stable and cool enough to allow the temperature of the collecting oceans to fall below boiling-point, and it is obvious that life cannot possibly have begun until after this had happened. [27] William Thomson: 'On the Secular Cooling of the Earth,' _Transact. R. S. Edinb._, xxiii., 1864, pp. 157-169.