The Rivers and Valleys of Pennsyh)ania. 219 



seen that the size of the present streams is not always a measure 

 of their previous importance, and to this we may ascribe the 

 difficulty that attends the attempt to decipher a river's history 

 from general maps of its stream lines. Nothing but a detailed 

 examination of geological structure and history suffices to detect 

 facts and conditions that are essential to the understanding of 

 the result. 



If the postulates that I shall use seem unsound and the argu- 

 ments seem overdrawn, error may at least be avoided by not 

 holding fast to the conclusions that are presented, for they are 

 presented only tentatively. I do not feel by any means abso- 

 lutely persuaded of the correctness of the results, but at the 

 same time deem them worth giving out for discussion. The 

 whole investigation was undertaken as an experiment to see where 

 it might lead, and with the hope that it might lead at least to 

 a serious study of our river problems. 



Part Fourth. The development of the rivers of Pennsylvania. 



23. Means of distinguishing between antecedent and adjusted 

 consequent rivers. — The outline of the geological history of Penn- 

 sylvania given above affords means of dividing the long progress 

 of the development of our rivers into the several cycles which 

 make up their complete life. We must go far back into the past 

 and imagine ancient streams flowing down from the Archean 

 land towards the paleozoic sea ; gaining length by addition to 

 their lower portions as the land grew with the building on of 

 successive mountain ranges ; for example, if there were a (Jam- 

 bro-Silurian deformation, a continuation of the Green Mountains 

 into Pennsylvania, we suppose that the pi-e-existent sti-eams must 

 in some manner have found their way westward to the new coast- 

 line ; and from the date of this mountain growth, it is apparent 

 that any streams then born must have advanced far in their 

 history before the greater Appalachian disturbance began. At 

 the beginning of the latter, as* of the former, there must have 

 been streams running from the land into the sea, and at times of 

 temporary elevation of the broad sand-flats of the coal measures, 

 such streams must have had considerable additions to their lower 

 length ; rising in long-growing Archean highlands or mountains, 

 snow-capped and drained by glaciers for all we can say to the 

 contrary, descending across • the Green Mountain belt, by that 

 time worn to moderate relief in the far advanced stage of its 



