The Rivers and Yalleys of P ennsyVoania. 191 



conclusion, it may perhaps be because the details of the geologi- 

 cal structure and development of Pennsylvania have not been 

 sufficiently examined. Indeed, unless the reader has already be- 

 come familiar with the geological maps and reports of the Penn- 

 sylvania surveys and is somewhat acquainted with its geography, 

 I shall hardly hope to make my case clear to him. The volumes 

 that should be most carefully studied are, first, the always inspir- 

 ing classic, "Coal and its Topography" (1856), by Lesley, in 

 which the immediate relation of our topography to the under- 

 lying structure is so finely described ; the Geological Map of 

 Pennsylvania (1856), the result of the labors of the first survey 

 of the state ; and the Geological Atlas of Counties, Volume X of 

 the second survey (1885). Besides these, the ponderous volumes 

 of the final report of the first survey and numerous rejDorts on 

 separate counties by the second survey should be examined, as 

 they contain many accounts of the topography although saying 

 very little about its development. If, in addition to all this, the 

 reader has seen the central district of the state and marvelled at 

 its even-crested, straight and zigzag ridges, and walked through 

 its narrow water-gaps into the enclosed coves that they drain, he 

 may then still better follow the considerations here presented. 



Part Second. Outline of the geological history of the region. 

 5. Cofiditlons of formation. — The region in which the Susque- 

 hanna and the neighboring rivers are now located is built in chief 

 part of marine sediments derived in paleozoic time from a lai'ge 

 land area to the southeast, whose northwest coast-line probably 

 crossed Pennsylvania somewhere in the southeastern part of the 

 state ; doubtless varying its position, however, by many miles 

 as the sea advanced and receded in accordance with the changes 

 in the relative altitudes of the land and water surfaces, such as 

 have been discussed by Newberry and Claypole. The sediments 

 thus accumulated are of enormous thickness, measuring twenty 

 or thirty thousand feet from their crystalline foundation to the 

 uppermost layer now remaining. The whole mass is essentially 

 conformable in the central part of the state. Some of the forma- 

 tions are resistent, and these have determined the position of our 

 ridges ; others are weaker and are chosen as the sites of valleys 

 and lowlands. The first are "the Oneida and Medina sandstones, 

 which will be here generally referred to under the latter name 

 alone, the Pocono sandstone and the Pottsville conglomerate ; to 

 these may be added the fundamental crystalline mass on which 



