Page:Popular Science Monthly Volume 36.djvu/235

Rh along the naked cliffs, he recognizes the results of mighty movements which the strata have undergone since they were formed, and will correctly ascribe their elevation to the same movements. The mountain has been produced through a doubling up caused by a sidewise pressure. If he turns his steps to the adjacent lowlands, he will make the same observation in horizontal strata. He now has his choice whether to believe in an elevation of a large part of the earth's crust, or in a sinking of the level of the sea since the crust was formed. One of the most debated questions of geology turns on this point.

The successive layers of the earth's crust have sometimes been compared to the leaves of a book. We read in them a long passage in the earth's history written by the scribe Nature herself while the events were happening, and therefore even more trustworthy than the sources of ordinary history. Yet many pages of the book are obscure, and those of the first part are still waiting to be deciphered; for, in the sparkling leaves of the archaic crystalline rocks, the letters that should give us knowledge of the beginnings of life on the earth seem to have been washed out. The first volume, telling of the Palæozoic age, makes us acquainted with a lower fauna, principally marine, from which only the vertebrates are absent. Then cartilaginous fishes appear in the Silurian and Devonian, and land-inhabiting vertebrates, amphibia, and reptiles in the Carboniferous and Permian. The development of life goes on in the Mesozoic epoch. The oldest and lowest organized mammals, the marsupials, meet us in the upper Trias. The Jurassic gives us the first birds, curious creatures with teeth in their bills and lizards' tails bearing feathers. Two specimens have been found, in the Archæopteryx, of the transition form between the reptile and the bird. The Cretaceous furnishes the first bony fish and new toothed birds, the odontornithides. The Cenozoic age, the fourth and last volume of the great book, exhibits another advance in the development of animal life; and in the Tertiary the forerunners of the present mammals, and in the diluvial, man, appear. A similar process of development from lower to higher forms is shown in the vegetable world.

The story of this gradual rise of more and more highly organized beings is certainly the most important content of those stone books. But, besides that, they record that the firm lands arose out of the floods, that the sea washed over the land, left it, and covered it again, while the mass of the land constantly grew. The pages that sketch the covering of a stretch of earth by the sea are fully written up; but the periods of dry land are more frequently made known by a gap than by a continuance of the relation; and, in the latter case, the terrestrial deposits are only present when the spot has been covered by a river or a lake.