INFLUENCE OF TOPOGRAPHY. 1217 



for that district. But for some districts not only is it necessary to solve the 

 problem of the present circulation of the underground water, but it is essen- 

 tial that possible past circulations should be studied. Thus, where there are 

 impervious strata which have been largely removed by erosion, the condi- 

 tions for circulation are very different before and after the removal of the 

 impervious stratum. To illustrate, it has already been pointed out that in 

 various districts in the lead and zinc region of the Mississippi Valley the 

 first concentration took place under conditions of artesian circulation, the 

 upper confining stratum being an impervious shale. Subsequent to that 

 time this shale has been removed from most of the region and a second 

 concentration has taken place by descending water under very different 

 conditions of circulation. 



CHARACTER OF TOPOGRAPHY. 



EFFECT OF VERTICAL ELEMENT. 



Where the relief is marked, the underground circulation is likely to 

 penetrate much deeper than in regions where the variations in relief 

 are slight. 



In mountainous and elevated plateau regions the lithosphere is likely 

 to have more numerous, larger, and deeper openings than low-lying areas. 

 Elevated areas are usually those of comparatively recent orogenic or 

 epeirogenic movement. Therefore, they are regions in which the rocks 

 have recently been fractured, and hence the processes of cementation 

 are less likely to" have closed the openings. In regions of very steep 

 topography the tendency for the material to glide down the slope under 

 the stress of gravity also tends to widen openings which have been once 

 formed. Such movements are known to be effective to the depth of 

 hundreds of meters. It is hence clear that elevated and rough regions 

 are those in which the underground circulation is likely to find large, 

 numerous, and deep openings. Further, elevated and mountainous reoions 

 are those in which the underground water has the greatest difference 'hi 

 head, and this is favorable to deep circulation. 



Thus, in mountainous regions, like the Cordilleras, it is to be expected 



that the underground circulation, both ascending and descending, is 



effective to greater depths upon the average than in regions of gentle 



topography. The Cordilleran region is well illustrated by the San Juan 



mon xlvh — 04 77 



