The Rivers and Yalleys of Pennsylvania. 231 



come to be ; and that the elevation that accompanied the tilting 

 was not so powerful in reversing the river to a northwest course 

 as the previous depression of the Newark basin had been in 

 turning it to the southeast. If the Anthracite did continue to 

 flow to the southeast, it may be added that the down-cutting of 

 its upper branches was greatly retarded by the decrease of slope 

 in its lower course when the monocline was formed. 



The only other method of reversing the original northwest- 

 ward flow of the streams that I have imagined is by capture of 

 their headwaters by Atlantic rivers. This seems to me less efi:'ec- 

 tive than the method just considered ; but they are not mutu- 

 ally exclusive and the actual result may be the sum of the two 

 processes. The outline of the idea is as follows. The long con- 

 tinued supply of sedimentary material from the Archean land on 

 the southeast implies that it was as continually elevated. But 

 there came a time when there is no record of further supply of 

 material, and when we may therefore suppose the elevation was 

 no longer maintained. From that time onward, the Archean 

 range must have dwindled away, what with the encroachment of 

 the Atlantic on its eastern shore and the general action of denud- 

 ing forces on its surface. The Newark depression was an effec- 

 tive aid to the same end, as has been stated above, and for a 

 moderate distance westward of the depressed belt, the former 

 direction of the streams must certainly have been reversed ; but 

 the question remains whether this reversal extended as far as the 

 Wyoming basin, and whether the subsequent formation of the 

 Newark monocline did not undo the effect of the Newark depres- 

 sion. It is manifest that as far as our limited knowledge goes, it 

 is impossible to estimate these matters quantitatively, and hence 

 the importance of looking for additional processes that may sup- 

 plement the effect of the Newark depression and counteract the 

 effect of the Newark uplift in changing the course of the rivers. 

 Let it be supposed for the moment that at the end of the Jurassic 

 uplift by which the Newark monocline was formed, the divide 

 between the Ohio and the Atlantic drainage lay about the middle 

 of the Newark belt. There was a long gentle descent westward 

 from this watershed and a shoi'ter and hence steeper descent east- 

 ward. Under such conditions, the divide must have been pushed 

 westward, and as long as the. rocks were so exposed as to open 

 areas of weak sediments on which capture by the Atlantic streams 

 could go on with relative rapidity, the westward migration of the 



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