CHAPTER IX 



THE REPTILES AND BIRDS 



A commentator on earth's history must be cautious to 

 avoid superlatives, and we have already spoken of the 

 Pennsylvanian period in extravagant terms. Yet in pass- 

 ing to the Permian period an antithetic emphasis can 

 scarcely be avoided, especially when even the cautious 

 historical geologist calls the transition 'one of the great 

 crises in the history of Ufe.' The end of the Pennsylvanian 

 was marked by the Appalachian revolution, which con- 

 tinued irregularly throughout the Permian, raising the 

 definitive Appalachian Mountains from Newfoundland 

 to Alabama and warping nearly all the Paleozoic forma- 

 tions into great folds— locally exhibited from the Blue 

 Ridge in Virginia to the AUeghenies in Pennsylvania by 

 remnants of mountains which, when young, must have 

 rivaled the present Himalaya. It is estimated, for ex- 

 ample, that the distance between Philadelphia and Al- 

 toona, Pennsylvania, now 177 miles, was shortened by 

 a third or more simply by folding and faulting as the 

 Appalachians were pushed into the air. The unstable 

 Appalachian 'trough,' which ever since the Cambrian 

 had subsided intermittently with every shudder of the 

 earth and had been repeatedly the site of marine inva- 

 sion, trapping some fifty thousand feet of sedimentary 

 strata, was now raised so high that the sea has never 

 invaded it again. 



