106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
winds are parallel to the coast, or if they strike the coast at a low angle, 
spits are built across the mouths of bays and estuaries. 
The materials for such spits may be the débris derived from the head- 
lands, or it may be brought down from the land by streams, or it may be 
swept along the coast for miles until it falls into the quieter waters 
behind or to the lee of the points where alone such spits can be formed. 
When the sea is shallow the coarser materials from the bottom — the 
continental shelf or shoulder — may be thrown upon the shore and swept 
along to help form these spits. Sometimes the spits entirely shut off the 
sea from the water of the bays, and a series of lagoons or salt lakes is 
formed along the shore. In a rainy region these lakes will become 
brackish, eventually fresh, and finally they will be silted up and form 
marshes and, in the end, dry land. The lakes of the coast of Alagóas 
are now passing through these stages, and at no distant day, speaking 
geologically, they will become extinct as lakes. 
Along certain coasts and about the mouths of large streams we have 
off-shore beaches formed in a somewhat different manner, though they 
usually merge into spits. These are sometimes called “ barrier beaches.” 1 
They are long, slender bars that barely rise above the surface of the water, 
In some cases they are many miles, or even hundreds of miles, in length. 
Such bars, beaches, or barriers are built up from the bottom. 
The explanation of barrier beaches was pointed out nearly two hun- 
dred years ago by a French engincer as “The limit between the em- 
bankments formed by storms at sea and those derived from fluviatile 
deposits.” 2 They are a part of the process of the outbuilding of ter- 
rigenous agencies, 
It is worthy of note that wherever these off-shore beaches exist the 
range of the tides is not great, and the currents set pretty constantly in 
one direction, and that in the submarine topography there is a rather 
sudden dropping off from a shallow shelf near shore to deep waters 
beyond. 
The straightness of such beaches is likewise noteworthy. Gannett 
says that “they are very straight, running for hundreds of miles with 
but slight deflections from a straight line.” 8 
1 N. S. Shaler, The physiography of the United States, p. 151, N. Y., 1897. 
Н. Gannett, Physiographic types. Typographic atlas of the United States, 
Folio I., U. 8. Geol. Surv. Washington, 1898. 
2 Quoted from Forfait, by Élie de Beaumont, Legons de géologie pratique, 
L, р. 326. 
* Henry Gannett, Topographic atlas of the United States. Physiographic 
types, р. 4. Washington, 1898, 
