104 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



IV. 



Conclusions regarding the Forms and Structure of the Stone 



Reefs. 



I. The Fokms of the Stone Reefs, 



It is often stated by writers on the stone reefs of Brazil that they are 

 perfectly straight. Liais himself, who saw those at and immediately 

 south of Pernambuco, says, '■^jamais la r'tcif ne se courhey To be very 

 exact, this is hardly true. 



That the reefs do curve somewhat is shown by the illustrations and 

 maps given herewith. These curves, however, are always gentle, and it 

 is only a matter of strict interpretation of language to say that they are 

 not straight. 



Inasmuch as the reefs are only modified sand beaches, in order to 

 understand their forms we must comprehend the forms of sea-beaches 

 generally, and also to some extent the causes of such forms as we have 

 along the Brazilian coast. 



Sand beaches, or beaches of construction, are built up with the mate- 

 rials supplied by their own coast-lines : cut from headlands, washed down 

 from the land by streams, or washed ashore by storm waves that throw 

 them up from the shallow sea off-shore. The idea that has been put for- 

 "ward that the sands of the northeast coast of Brazil md.y have been 

 brought from the west coast of Africa may safely be set aside as quite 

 out of the question. 



Straight beaches may be produced in two ways : — 



I. By the elevation of a sea bottom, or the depression of a land sur- 

 face that is so smooth that the new beach line, after elevation or depres- 

 sion, is unbroken by surface irregidarities. 



II. By the natural process of straightening an originally irregular, 

 indented, or broken shore-line by the cutting down of the headlands and 

 the choking up of embay ments. 



In the latter case the beach is crooked at first and its straightness 

 comes only with age. 



1 L'Espace Cdeste, p. 546. 



