and emergences of the great continental plateaus were equally numerous in the two great periods, though they were probably, with the exception of the latest one of each, more complete in the older period. In so far, then, as deposition of sediment is concerned, these considerations would scarcely lead us to infer that it was more rapid in the Palæozoic. But the Palæozoic sediments may be estimated in the aggregate at about 50,000 feet in thickness, while those of the Mesozoic scarcely reach 8,000. We might, therefore, infer that the Palæozoic period was perhaps five or six times as long as the Mesozoic. If we take the second class of rocks, the limestones, and suppose these to have been accumulated by the slow growth of corals, shells, etc., in the sea, we might, at first sight, suppose that Palæozoic animals would not grow or accumulate limestone faster then their Mesozoic successors. We must, however, consider here the probability that the older oceans contained more lime in solution then those which now exist, and that the equable temperature and extensive submerged plateaus gave very favourable conditions for the lower animals of the sea, so that it would perhaps be fair to allow a somewhat more rapid rate of growth of limestone for the Palæozoic. Now the actual proportions of limestone may be roughly stated at 13,000 feet in the Palæozoic, and 3,000 feet in the Mesozoic, which would give a proportion of about four and a quarter to one; and as a foot of limestone may be supposed on the average to require five times as long for its formation as a foot of sediment, this would give an even greater absolute excess in favour of the Palæozoic on the evidence of the limestones an excess probably far too great to be accounted for by any more favourable conditions for the secretion of carbonate of lime by marine animals. The data for such calculations are very uncertain, and three elements of additional uncertainty closely related to each other must also be noticed. The first is the unknown length of the intervals in which no deposition whatever may have been taking place over the areas open to our investigation. The second is the varying amounts in which material once deposited may have been swept away by water. The third is the amount of difference that may have resulted from the progressive change of the geographical features of our continents. These uncertainties would all tend to diminish our estimate of the relative length of the Mesozoic. Lastly, the changes that have taken place in living beings, though a good measure of the lapse of time, cannot be taken as a criterion here, since there is much reason to believe that more rapid changes of physical conditions act as an inducing cause of rapid changes of life.