THE TRAVELS OF THE UNDERGROUND WATER 189 



Mile*. 



Elsewhere the karst river may emerge from its subterranean 

 course in a broader depressed area bounded by vertical cliffs, from 

 which it later disappears beneath the limestone wall. Such de- 

 pressions of the karst are known as poljen, and appear in most 

 cases to be above the downthrown blocks in the intricate fault 

 mosaic of the region. Some of these steeply walled inclosures 

 have an area of several hundred square miles, and especially at 

 the time of the spring snow melting they are flooded with water 

 and so transformed into seasonal lakes (Fig. 199 and p. 422). It 

 appears that at such times the cave 

 galleries of the region with their local 

 narrows are not able to carry off all 

 the water which is conducted to them ; 

 and in consequence there is a tempo- 

 rary impounding of the flood waters in 

 those portions of the river's course 

 which are open to the sky and more 

 extended. The rush of water at such 

 times may bring the red clay into the 

 subterranean channels in sufficient 

 quantity to clog the passages. The 

 Zirknitz Lake usually has high water 

 two or three times a year, and exceptionally the flooding has con- 

 tinued for a number of years. It has thus in some districts been 

 necessary to afford relief to the population through the construc- 

 tion of expensive drainage tunnels. 



The conditions which are typified in the Karst area to the east 

 of the Adriatic Sea are encountered also in many other lands; as, 

 for example, in the Vorarlberg and Swiss Alps, in Lebanon, and 

 in Sicily. 



The return of the water to the surface. Water which has de- 

 scended from the surface and been there held between impervious 

 layers, may be under the pressure of its own weight or " head " ; 

 and will later find its way upward, it may be to the surface or 

 higher, where a perforation is discovered in its otherwise imper- 

 vious cover. Such local perforations are produced naturally by 

 lines of fracture or faulting (widened at their intersections), 

 and artificially through the sinking of deep wells. The water, 

 which at ordinary times reaches the surface upon fissures, is usually 



FIG. 199. The Zirknitz seasonal 

 lake within a polje of the Karst 

 (after Berghaus). 



