CHAPTER XIX 

 THE PERMIAN PERIOD 



A transition period. The Permian marks the transition 

 from the Paleozoic era to the Mesozoic. In eastern United 

 States the Permian rocks are a mere continuation of the Penn- 

 sylvanian system, while in the Rocky Mountains it is often 

 impossible to separate Permian strata from those of the Trias- 

 sic period. Only locally are the systems sharply marked off 

 from each other. 



Emergence of the eastern region. --Throughout the Per- 

 mian, the interior sea was slowly being withdrawn. The 

 deposition of the Coal Measures continued on into the early 

 Permian in the Ohio Valley. Some of the sediments were 

 laid down in rivers or fresh-water lakes, and contain abundant 

 leaves of plants. Later in the period the region seems to 

 have been drained, leaving a broad lowland which was only 

 feebly eroded. 



The sea lingers in the Southwest. In the southern part 

 of the tract which we now call the Great Plains the sea 

 lingered somewhat later. Shales, sandstones, and limestones 

 quietly accumulated in Texas, and perhaps in Kansas. 



The red beds. The later Permian rocks of northern 

 Texas, however, tell of very different conditions; they are 

 reddish shales, with layers of gypsum and rock salt. Evi- 

 dently the sea had by that tune receded still farther, leaving 

 a desert region in western United States, with saline lakes 

 in the depressions. In Colorado and some other places the 

 formation of these red beds began during the Pennsylvanian 

 and continued on into the Triassic period, indicating that the 

 arid climate was of long duration. 



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