178 
PACIFIC SCIENCE, Vol. I, July, 1947 
nearer to the shore line are much the shorter 
and that under fluctuating conditions of un¬ 
balance, water movement in larger openings 
across the zone of contact would take place 
here more rapidly than elsewhere (see Fig. 
2). Along a shore unprotected by a cap rock, 
the thin edge of the Ghyben-Herzberg lens 
would be especially vulnerable to disturb¬ 
ance or destruction during marked fluctua¬ 
tions. There, particularly, the adverse effect 
of large fissures or other irregularity of per¬ 
meability is certain to be great. 
From these considerations, the value of a 
cap rock as a barrier between fresh and salt 
water is readily seen. As stated elsewhere, 
the interposition of the cap rock between 
fresh and salt water in effect completes the 
physical pattern of a U-tube. It tends to 
raise the head of basal water and to truncate 
and thicken its shore margin. This barrier 
has the result of eliminating the thin edge 
of the lens, with its dangerous sensitivity to 
plus and minus fluctuations. It should not 
be overlooked, also, that in most places the 
Ghyben-Herzberg condition is first recog¬ 
nized and is most useful in the shore zone 
where the water is most accessible and often 
most needed. It is possible that more 
complete and more extensive exploration 
will demonstrate the interior existence of 
Ghyben-Herzberg lenses in some islands 
where the condition is not well shown at the 
shore; such discoveries would confirm the 
contentions of this paper. 
PARTS OF THE LENS 
In the preceding sections, the five factors 
controlling the establishment and growth of 
the Ghyben-Herzberg lens have been dis¬ 
cussed. Attention is now directed to the 
parts of the lens and to the nature of its 
lower boundary. It has been accepted that 
the lower surface of the lens is a zone of 
transition from fresh to salt water, and that 
since the two liquids are miscible the zone 
will have thickness. The perfect condition 
of a sharp boundary can only obtain with 
immiscible liquids. If the permeability is 
too great for the amount of infiltration, or 
if there is great irregularity of permeability, 
the mixing and mutual interpenetration of 
the two waters will be promoted and the 
zone of transition will be thickened. Such 
mixing may go so far that no part of the 
lens is free from salt contamination. 
GROWTH OF DIFFUSION ZONE 
Another effect, that of fluctuation or alter¬ 
nate movements of the zone of transition, 
may not be so readily discerned. With im¬ 
miscible liquids and no matrix such as rock, 
the contact surface would move up or down 
according to relative pressures and with little 
or no deformation. However, in rock with 
miscible liquids, the migration of the zone 
downward into rock formerly filled with salt 
water would first involve driving out some 
of the salt water. But it would also result in 
assimilation of some of the salt water re¬ 
maining longer in smaller openings. If 
such a process continued, the water composi¬ 
tion at any one place would tend to approxi¬ 
mate more and more that of the advancing 
water and retain less and less of the quality 
of the water originally displaced. However, 
there would commonly, after any short time, 
be some residue of the displaced water. If 
we assume that at any given time the local 
composition is a function of the composi¬ 
tions of the two waters and of the time dur¬ 
ing which Water A has moved into the realm 
of Water B, a process analogous to the 
exponential law of rinsing, some interesting 
consequences appear. If, for example, the 
two waters have an initially sharp boundary, 
and that boundary is moved under hydro¬ 
static changes of sign from one realm to the 
other, in equal amounts successively, the 
most immediate effect is the spreading of 
the boundary so that it is no longer sharp but 
assumes a graded transition form. 
