Abbe.] 490 IMarchr,, 
Tlio bars servo in general to protect from the sea tlie nnicli more 
irregular and indented shore-line of the mainland, while the}' 
themselves present a much simpler eoast-line. 
Behind these bars we generally find a system of tidal marshes 
or shallow lagoons of varying width, but rarely exceeding 20 or 
25 feet in depth. The sandy beaches outside are worthless for 
cultivation, but the marshes yield a coarse salt hay prized by 
cattle ; while with the aid of some canals the lagoons have been 
converted into inland waterways that afford safe passage to small 
vessels bound up or down the coast. 
It will l)e interesting then to study the formation of these 
natural breakwaters, more particularly since our special study is 
of those dangerous Caj^es, Hatteras, Lookout, and Fear. 
I'^orynatiori of the bars. — Wherever these bars occur we also 
find a broad, comparatively shoal bench of detritus bordering 
the coast- line (on the east). Over this shelf the water grad- 
ually deepens to about 100 fatlunns, after which the descent is 
much more rapid to the oceanic depths. 
The great Atlantic rollers rolling in over this bench or con- 
tinental shelf, and stirring up the bottom, carry wnth them the 
material thus Avoi-ked on. Part of this detritus is thrown up 
hy the waves where the}- break and thus forms a bar, while 
the other portion is carried out by the undertow and serves to 
shoal up the deeper water. This shoaling of the deeper seaward 
waters causes the waves of the next storm to break a little 
further out. Thus there will l)e built out a series of l)ars until 
a depth is reached where the waves no longer stir up the 
bottom but work directly and wholly on the beach. From 
this stage onward the waves are working to cut away the 
bars and force the coast-line shoreward. Just enough waste 
will be carried seaward to keep the submerged slope at an angle 
up which the waves maj^ run without breaking until they reach 
the beach. This line along which the waves first acted, and which 
later was perhaps moved seaward for a time, but now again 
moves landward, may be called the locus of maximum activity 
of the waves. Plainly this dine is not a fixed one, but must 
vary with the size of the waves, depth of water, and composition 
of the bottom. 
