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 debris 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 fi'om 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 Alagoas 

 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 diflerent manner, though they 

 usually merge into spits. These are sometimes called " barrier beaches."^ 

 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 banners are built up from the bottom. 



The explanation of barrier beaches was pointed out nearly two hun- 

 dred years ago by a French engineer as '' The limit between the em- 

 bankments formed by storms at sea and those derived from fluviatile 

 deposits." ^ 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 

 onfi direction, and that in the submarine topography there is a rather 

 sudden dropping .off from a shallow slielf 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." ^ 



1 N. S. Shaler, The physiography of the United States, p. 151, N. Y., 1897. 



H. Gannett, Physiographic types. Typograpliic atlas of the United States, 

 Folio L, U. S. Geol. Surv. Washington, 1898. 



- Quoted from Forfait, by ifclie de Beaumont, Lefons de geologie pratique, 

 I., p. 326. 



8 Henry Gannett, Topographic atlas of the United States. Physiographic 

 types, p. 4. Washington, 1898, 



