branner: the stone reefs of brazil. 105 



Inasmuch as the beaches of which the reefs have been made have the 

 appearance of being accumulations or accretions, we may set aside the 

 probability of their having been formed by the elevation of a smooth sea 

 bottom and deal with the methods by which straight beaches are pro- 

 duced from crooked ones. 



They may be considered as originating in two ways, or by a single 

 structural process, operating under two different conditions. These con- 

 ditions may produce : — 



First, shore beaches, — the accumulations along the margin between 

 the mainland and the main body of water. 



Second, off-shore beaches, or the beaches developed as spits, bars, or 

 barrier beaches lying parallel with the main coast-line. 



The shore beach.. — The form of a shore-line is determined (1) by wave 

 action, (2) by the resistance of the shore rocks to marine encroachment, 

 and (3) by the direction of inshore currents. On a coast having alter- 

 nate bays or estuaries and headlands, we inevitably have alternate cur- 

 rents and eddies next to the shore. The waves and currents attack the 

 headlands and throw the coarse materials cut from them into the more 

 quiet waters of the bays, where they sink to the bottom. If, however, 

 the prevailing winds drive the waves squarely against the shores, the silts 

 are thrown back against the inner margins of the bays, and the beaches 

 are built outwards by a continuous and approximately uniform process 

 of accretion. The tendency along such shores is, therefore, for the 

 beach gradually to become straight by the cutting down of the headlands 

 and the outbuilding of the concave beaches. The early beaches must, 

 therefore, have been crooked, or more so, at least, than the later ones. 

 The temporary shiftings of the currents, either of the streams or of the 

 sea, have probably played an important part in determining the forms and 

 locations of the beaches and of the stone reefs. 



It is evident, however, that the straight beaches are comparatively 

 mature ones, for it is only after the agencies mentioned have been in 

 operation for a long time that the broken line of the early history of the 

 shore becomes a straight one.^ 



The off-shore beach. — Off-shore beaches might be divided into those 

 originating as spits and those built up from the bottom as bars not 

 connected at the surface with the land. No sharp line can be drawn 

 between them, however. If the near-shore currents or the prevailing 



1 Lapparentsays, "the work of the sea on flat coasts is essentially constructive." 

 (Traite de geologic, 2me ed. 171.) This statement, however, might be turned 

 about and be equally true; tliat is, a newly constructed sea-coast is essentially flat. 



