440 THE PERMIAN PERIOD 



Carboniferous. It is, however, not at all impossible that the thin 

 Permian may have been stripped away by denudation, as it has 

 been over nearly all of the northern plateau of Arizona. 



At the time when the eastern part of the Great Basin was thus 

 converted into land, the ancient land area of its western border 

 was depressed beneath the sea. It is probable that these two 

 movements were connected, though they may have been sepa- 

 rated by a considerable interval of time. In Nevada west of 

 1 1 7° 30' W. longitude no Palaeozoic rocks have been found, and 

 the Trias rests directly upon the Archaean. 



However they may be explained, the geographical revolution 

 which closed the Palaeozoic era was accompanied by the most 

 profound and far-reaching changes which have ever occurred in 

 the recorded history of life, after which we find ourselves in a new 

 world. It is probable that the change was a relatively rapid one, 

 but there are sufficient connections between the faunas and floras 

 of the two eras to show that the later were derived from the 

 earlier, and that the gaps are due to the imperfections of the 

 record. 



