The duration of the Cretaceous subsidence must have been very great. We do not know the rate at which the Foraminifera accumulate calcareous mud. In some places, where currents heap up their shells, they may be gathered rapidly; but on the average of the ocean bed, afoot of such material must indicate the lapse of ages very long when compared with those of modern history. We need not wonder, therefore, that while some forms of deep-sea Cretaceous life, especially of the lower grades, seem to have continued to our time, the inhabitants of the shallow waters and the land have perished; and that the Neozoic or Tertiary period introduces us to a new world of living beings. I say we need not wonder; yet there is no reason why we should expect this as a necessary consequence. As the Cretaceous deluge rose over the continents of the Mesozoic, the great sea saurians might have followed. Those of the land might have retreated to the tracts still remaining out of water, and when the dry land again appeared in the earlier Tertiary, they might again have replenished the earth, and we might thus have truly been living in the Reptilian age up to this day. But it was not so. The old world again perished, and the dawn of the Tertiary shows to us at once the dynasties of the Mammalian age, which was to culminate in the introduction of man. With the great Cretaceous subsidence the curtain falls upon the age of reptiles, and when it rises again, after the vast interval occupied in the deposition of the green-sand and chalk, the scene has entirely changed. There are new mountains and new plains, forests of different type, and animals such as no previous age had seen.
How strange and inexplicable is this perishing of types in the geological ages! Some we could well spare. We would not wish to have our coasts infested by terrible sea saurians, or our forests by carnivorous Dinosaurs. Yet why should these tyrants of creation so utterly disappear without waiting for us to make war on them? Other types we mourn. How glorious would the hundreds of species of Ammonites have shone in the cases of our museums, had they still lived! What images of beauty would they have afforded to the poets who have made so much of the comparatively humble Nautilus! How perfectly, too, were they furnished with all those mechanical appliances for their ocean life, which are bestowed only with a niggardly hand on their successors! Nature gives us no explanation of the mystery.