THE STRUGGLE OF SEA AND LAND. 223 



along the naked cliffs, he recognizes the results of mighty move- 

 ments 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 low- 

 lands, 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 pas- 

 sage in the earth's history written by the scribe Nature herself 

 while the events were happening, and therefore even more trust- 

 worthy 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 crys- 

 talline 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 Palaeozoic age, makes us ac- 

 quainted 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, am- 

 phibia, and reptiles in the Carboniferous and Permian. The de- 

 velopment 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 Archceopteryx, 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 organ- 

 ized 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. 



