548 



KIRK BRYAN 



Similar masses of pervious rock may be bounded on one or more 

 sides by impervious rock brought into place by faulting. Overflow 

 will then take place among the fault line, as is exemplified in the 

 northern Vosges Mountains, France^ (Fig. 15). 



In arid regions, particularly, great thicknesses of alluvium have 

 accumulated on irregular rock floors, and if the irregularities of 



On^/- Spring 



^Spring 



Fig. 14. — Cross-section of the North Downs, near London, England: i, gault; 

 2, upper greensand; 3, chalk marl.; 4, main chalk, pervious to water; 5, lower London 

 Tertiary strata; 6, London clay, impervious to water. (Redrawn from Woodward.) 



Fig. 15. — Cross-section showing overflow spring in the northern Vosges Moun- 

 tains, France. H. Hauptbunt-sandstein (porous); P. Rotelschiefer (impervious). 

 (After Leppla.) 



the rock floor project through the surface of the alluvial plains, 

 the water contained in the alluvium may be forced to the surface. 

 Essentially similar conditions may be brought about by intrusion 

 of igneous material into such alluvial plains. Under these condi^ 

 tions the rock projections act as dams against the regular flow of 

 the ground water, and the resulting springs may be called rock 



' A. Leppla, quoted by K. Keilhack, Lehrbuch der Grundwasser und Quellenkunde, 

 Berlin, 191 2. 



