All this time we have been hunting for parts of “the puzzle” in those ancient oolite rocks between the coal and the chalk, and those we have found are very important. We have seen the slow progress from simple sea shells to simple fishes, and then onwards to fish-lizards and bird-reptiles with one little marsupial animal, of a far higher kind, in between, as if to tell us beforehand what more complete and perfect animals we might expect by-and-by. After the fishes we have found fish-lizards, then bird-reptiles with wings, but no feathers, and later still a bird-reptile with wing and tail feathers. How different the life of the Earth was at the end of the “reptile age” of the oolite rocks, to the far back Laurentian time when one little creature, our old friend eozöon, alone held possession of the seas! THE CRETACEOUS PERIOD. Now let us look into the rocks next above, and see what is to be found there. We have arrived in the Cretaceous period, or time when the chalk was formed.[13] You remember I told you you might call this “foraminifera earth” because so much of it was made up of the shells of these tiny animals, thousands of which could be put into a thimble. Whenever you make a mark with a piece of drawing chalk you rub off a number of them, and you will see what pretty little creatures they were if you look at the drawings of some of them on the next page as they are seen under the microscope, magnified thousands of times their natural size; but there are others of different shapes. On the same page too there is a handsome shell, called an ammonite, and of its real size, common in chalk rocks. The seas of the time must have been very deep as I have explained before, and the chalk contains numbers of bones of fishes everywhere, and many of the remains of the reptile-like creatures of the time before. Corals, sea-urchins, crabs, &c., abounded, and as you can scarcely ever see chalk without immense flint stones in it, you may suppose what millions of sponges lived on the rocks, for these flints are partly made up of their fossil bodies.[14] Another Cretaceous period is beginning now at the bottom of the Atlantic and Pacific Oceans, where it is deep enough to cover the Alps, for these little foraminifera are living on the surface in countless millions, and day by day their fossil shells are settling down to the bottom and forming a soft grey mud, full of the carbonate of lime like chalk. The climate of the Cretaceous age was mild and pleasant, as we know from the kind of animals in the seas. Slowly the water began to get shallower and shallower by the upheaval of the bed, and at last the bottom of this mighty chalk ocean came up to the light and sun, to be covered in some places with the drift and worn particles of older rocks swept over it by rivers, and to receive new plants and new animals, and in some places to remain almost bare, as it is on the downs of Brighton. [Illustration: VII. FOSSILS OF THE CHALK.