AND ITS INHABITANTS 65 



The third era of geologic time, the Paleozoic, is based upon 

 a thickness of about 4.75 miles of muds and sands and 3.4 

 miles of limestones. These thicknesses are in the Appalachian 

 region, and form one of the most complete of all known 

 Paleozoic records. In the Cordilleran area, where the record 

 is far less complete, there are about 2 miles of coarse deposits 

 and 3 miles of limestones. Toward the medial regions of the 

 continent the thick lateral sections thin down to about i mile, 

 essentially of limestones. 



Shortly after the opening of the Paleozoic appear the fishes, 

 the first of the vertebrates, and now, surprising as it may seem, 

 in the fresh waters of the land and not in the seas. Before half 

 of the Paleozoic is gone, double-breathing is perfected, and 

 the amphibia begin to people the dry lands, and long before 

 the close of the era give rise to the reptiles. It is also in the 

 middle Paleozoic that the most primitive land floras arise, 

 and not long afterward, in the Coal Measures, their remains, 

 falling into great swamps, create the world's greatest supply 

 of fuel in the soft and hard coals (see Fig. 10). 



The fourth era, known as the Mesozoic, has, in western 

 North America, about 7.5 miles of sediments, of which not 

 more than 1.25 miles are limestones. On the other hand, in 

 the lands bordering the Gulf of Mexico and chiefly in Mexico 

 there are less than 3 miles of Mesozoic sediments, but here 

 the limestones make up more than one-half of the total 

 thickness. 



The Mesozoic is the Age of Reptiles, and yet the little 

 mammals and the toothed birds are storing up intelligence and 

 strength to replace the reptiles when the cycads and conifers 

 shall give way to the higher flowering plants. Then follows 

 the Cenozoic era, the Age of Mammals, with a thickness in 

 California of about 5 miles of coarse sediments, contrasting 

 with about 4 miles of fresh-water detritals in the Rocky 

 Mountain area, and in these rocks occurs a most wonderful 



