A HISTORY OF SCIENCE 



ages, invertebrates brachiopods and crinoids and 

 cephalopods were the dominant types. But very 

 early no one knows just when there came fishes of 

 many strange forms, some of the early ones enclosed 

 in turtle-like shells. Later yet, large spaces within the 

 interior sea having risen to the surface, great marshes 

 or forests of strange types of vegetation grew and de- 

 posited their remains to form coal-beds. Many times 

 over such forests were formed, only to be destroyed by 

 the oscillations of the land surface. All told, the strata 

 of this Paleozoic period aggregate several miles in thick- 

 ness, and the time consumed in their formation stands 

 to all later time up to the present, according to Pro- 

 fessor Dana's estimate, as three to one. 



Towards the close of this Paleozoic era the Appa- 

 lachian Mountains were slowly upheaved in great con- 

 voluted folds, some of them probably reaching three or 

 four miles above the sea-level, though the tooth of time 

 has since gnawed them down to comparatively puny 

 limits. The continental areas thus enlarged were 

 peopled during the ensuing Mesozoic time with multi- 

 tudes of strange reptiles, many of them gigantic in size. 

 The waters, too, still teeming with invertebrates and 

 fishes, had their quota of reptilian monsters ; and in the 

 air were flying reptiles, some of which measured twenty- 

 five feet from tip to tip of their batlike wings. During 

 this era the Sierra Nevada Mountains rose. Near the 

 eastern border of the forming continent the strata were 

 perhaps now too thick and stiff to bend into mountain 

 folds, for they were rent into great fissures, letting out 

 floods of molten lava, remnants of which are still in 

 evidence after ages of denudation, as the Palisades 



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