The Rivers and Yalleys of Pennsylvania. 193 



7. Camhro- Silurian and Permian def ordinations. — This great 

 series of once horizontal beds is now wonderfully distorted ; but 

 the distortions follow a general rule of trending northeast and 

 southwest, and of diminishing in intensity from southeast to 

 northwest. In the Hudson Valley, it is well known that a con- 

 siderable disturbance occurred between Cambrian and Silurian 

 time, for there the Medina lies unconformably on the Hudson 

 River shales. It seems likely, for reasons that will be briefly 

 given later on, that the same disturbance extended into Pennsyl- 

 vania and farther southwest, but that it affected only the south- 

 eastern corner of the State ; and that the unconformities in 

 e"vidence of it, which are preserved in the Hudson Valley, are 

 here lost by subsequent erosion. Waste of the ancient land and 

 its Cambro-Silurian annex still continued and furnished vast beds 

 of sandstone and sandy shales to the remaining marine area, 

 until at last the subsiding Paleozoic basin was filled up and the 

 coal marshes extended broadly across it. At this time we may 

 picture the drainage of the southeastern land area wandering 

 rather slowly across the great Carboniferous plains to the still 

 submerged basin far to the west ; a condition of things that is 

 not imperfectly represented, although in a somewhat more 

 advanced stage, by the existing drainage of the mountains of the 

 Carolinas across the more modern coastal plain to the Atlantic. 



This condition ^as interrupted by the great Permian deforma- 

 tion that gave rise to the main ranges of the Appalachians in 

 Pennsylvania, Virginia and Tennessee. The Permian name seems 

 appropriate here, for while the deformation may have begun at 

 an earlier date, and may have continued into Triassic time, its 

 culmination seems to have been within Permian limits. It was 

 characterized by a resistless force of compression, exerted in a 

 southeast-northwest line, in obedience to which the whole series 

 of Paleozoic beds, even twenty or more thousand feet in thickness, 

 was crowded gradually into great and small folds, trending north- 

 east and southwest. The subjacent Archean terrane doubtless 

 shared more or less in the disturbance : for example. South 

 Mountain is desci'ibed by Lesley as "not one mountain, but a 

 system of mountains separated by valleys. It is, geologically 



considered, a system of anticlinals with troughs between 



It appears that the South' Mountain range ends eastward [in 

 Cumberland and York Counties] in a hand with five [anticlinal] 

 fingers."* 



* Proc. Amer. Phil. See, xiii, 1873, 6. 



