388 DYNAMICAL GEOLOGY. 



tain ranges, the reasons for the directions of drainage courses over such 

 regions are easily understood. The prevailing courses are longitudinal as 

 regards the range ; not because synclinal troughs are longitudinal, for these, 

 in the case of bold flexures, are not ordinarily the courses of river valleys ; 

 but for the more general reason that the flexures and faults in the range are 

 longitudinal. The greater valleys are made along anticlines, because of 

 the profound longitudinal fracturing of their summits, in consequence of 

 the tension produced by the upward bending of the strata. This leaves the 

 intervening synclinal belt as the course of the mountain ridges. Besides, 

 the synclinal strata come under extreme pressure during the flexing process, 

 and may have derived by this means greater durability. If the rocks of the 

 range are crystalline schists and limestone, the limestone yields easily to 

 denudation, and would determine in general the course of the drainage 

 channel. But among uncrystalline rocks, limestone is harder than shale and 

 some sandstone. 



It has been stated that in a region of upturned rocks, as that of the 

 Appalachian Kange, the flexures are made in series along a few parallel 

 lines, and sometimes in a succession of groups ; and consequently that those 

 of different lines often overlap at their extremities. Hence, along these 

 intervening or overlapping portions the strata are irregularly warped and 

 fractured, and thus weakened. Here, consequently, erosion should be easy, 

 and transverse or oblique courses of drainage would result. 



Great mountain ranges and systems have been shown to have one or 

 more curves in their courses. The Appalachian Range, for example, changes 

 from its south-by-west course in New York to west-southwest in Pennsyl- 

 vania, and then leaves this state with a south-southwest course, which to the 

 southward veers again to west-southAvest. Here is another cause for trans- 

 verse lines of drainage ; for such a range usually diminishes in height over 

 its more nearly meridional or more latitudinal part. In the Appalachians 

 the lower part is along the latter ; and here, as Lesley's map of Pennsyl- 

 vania shows (page 730), the range is crossed by the Susquehanna. 



Finally, along a region of a number of close-pressed folds, having great 

 longitudinal fractures with displacements, a drainage valley may take great 

 width. 



If the plications or monoclines over an extended area have small dip, 

 then the broad synclines and the depression between monoclines or lines of 

 displacement may become the courses of streams. 



Epeirogenic movements that give a height of many thousands of feet to 

 large continental areas add these thousands to the elevation of the moun- 

 tain ranges along the region; and hence, besides causing flows of water down 

 the gentle slopes, they produce a vast increase of precipitation and denuda- 

 tion about the summits, and make the streams great rivers. Over the in- 

 terior of continents such movements may cause undulations or warpings 

 of the surface, which occasionally reverse the flow of rivers, or unite inde- 

 pendent river systems into one, or make depressions that become the basins 

 of lakes. 



