tit wehbe Cr OF SEA BARRIERS UPON ULTIMATE 
DRAINAGE 
THE causes which determine the location of river courses in 
the neighborhood of their discharge into the sea, where the cur- 
rents are slow and their power of erosion small, are often quite 
insignificant. 
If, however, a stream once becomes established in any given 
course, and the region through which it flows becomes elevated, 
its sluggish current at once becomes active and forms a valley 
of greater or less depth. Its tendency through subsequent 
changes in the land level is to remain in the valley approximately 
as originally formed. This tendency is especially strong if this 
original valley is parallel with the strike of the strata. 
It is not the purpose of the present paper to discuss the 
development of intricate drainage systems along structural lines, 
and through long periods of time, but simply to suggest that a 
drainage system may sometimes have a portion of its course fixed, 
by spits and barrier beaches along the coast line, at the same 
time that the sediments which are to form the rocks of its future 
drainage area are being deposited ; and also that the drainage 
when established thus early may remain more or less fixed 
through its subsequent history. 
Along coast lines generally, and especially along those of 
gently sloping coastal plains, spits, bars, and barriers are more 
or less common. For our present purposes these may all be 
spoken of as barriers, and so far as the present paper is concerned 
it does not matter whether they are composed of sand, gravel, 
or coral; neither do the forces by which they are built up need 
to be discussed. 
The lagoons between the barriers and the shore vary in 
length with the barriers, from a few hundred yards to many 
miles. Such lagoons are parallel to the shore and_ usually 
445 
