branner: the stone reefs of brazil. 



171 



VI. 



The Consolidation of the Stone Reefs. 



PAOE 



The cement 171 



Analysis of the rock 172 



Microscopic examination . . . 173 



Origin of the cementing material 174 



I. Cement from beach sands 



by rain-water or spray . 175 

 II. Lime carbonate from the 



ocean .177 



C02 of volcanic origin . . 178 



Coo from sea-water . . . 182 



Is the process universal ? . 184 



III. Lime carbonate from the 



land 186 



Consolidated beaches of 



the Levant 187 



Relations of density to deposition 190 



IV. The seaward percolation 



of acid land-water . . 192 

 Possible influence of cli- 

 mate 193 



The process not a continu- 

 ous one 194 



Conclusions 195 



Having studied the forms and origin of the beaches, we may 

 now consider the process or processes by which they may have been 

 consolidated. 



That the ancient forms of the beaches of which the stone reefs are 

 made have been preserved is due to the fact that the sands of these 

 beaches have been firmly cemented, ^yithout the hardening, we should 

 have had no stone reefs, because the processes of beach changes would 

 have failed to leave these old shore lines outstanding : they would 

 either have been buried by later accumulations, or have been destroyed 

 by wave action. 



The Cement. 



The hardening of the rock is due to the deposition of carbonate of 

 lime in the interstices of ordinary beach sands. This is shown by a 

 mici'oscopic examination of fresh specimens of the rock (see page 173), 

 and by putting a piece of the rock in an acid that will remove the lime 

 carbonate. 



A chemical analysis has been made of a sample of the Rio Formoso 

 reef rock, with the following results : — 



