286 J. S. Brown — Relation of Sea Water 



approached a sinusoid. It appears that as the permea- 

 bility of the rocks increases the curve flattens. Thus 

 Sanf ord has described the body of fresh water above the 

 salt water in the fractured limestones of the Florida 

 Keys, as a "thin sheet" (see p. 279), and Lindgren 11 has 

 found practically the same condition in the very porous 

 basalts of Molokai. This is a very natural result, for 

 the greater freedom of circulation permits the water table 

 to sink lower, and the necessity for equilibrium then 

 compels the salt water to rise higher, so that the two 

 approach each other. On coasts composed of porous 

 sand the conditions are probably somewhat as shown in 

 figure 4 B. The general zone of contact slopes downward 

 beneath the coast until it is interrupted by impervious 

 rocks. 



In Connecticut this invasion of the land by sea water 

 terminates within a few hundred feet of the shore, owing 

 to the fact that the underlying crystalline rocks become 

 practically impervious within a few hundred feet of the 

 surface. Pennink, 12 however, has demonstrated most 

 convincingly that salt water extends for several miles 

 inland beneath the coast of Holland and similar condi- 

 tions probably exist at other places. The west coast of 

 Holland consists of a belt of dunes about 5 miles wide 

 and at its crest 10 to 20 meters above sea level, which 

 separates the sea from a belt of polders. 13 The polders 

 were created by draining the sea of Haarlem about 1850 

 and lie from 4 to 6 meters below sea level. In seeking 

 to supplement the Amsterdam water supply a line of 

 test wells was driven across this belt of dunes. Chloride 

 determinations were made on the ground water at many 

 different depths in these wells, and a cross section show- 

 ing the actual contact between fresh water and under- 

 lying salt water was determined. (Figure 5.) 14 Certain 



11 Lindgren, Waldemar : The water resources of Molokai, U. S. Geol. Sur- 

 vey Water-Supply Paper 77, pp. 26-47. 1903. 



12 Pennink, J. M. K. : De " prise d 'eau ' ' der Amsterdamsche duin water- 

 leiding, Tijdschrift van het Koninglijk Instituut van Tngenieurs, pp. 183- 

 238, The Hague, 1904. 



13 In Holland and Belgium the term polder is applied to a tract of marshy 

 land, lower than the sea, which has been diked and reclaimed to cultivation. 

 — Standard Dictionary. 



14 The zero of Amsterdam shown in Pig. 5 is approximately 0.2 meter 

 above mean sea level. 



