194 BULLPITIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Changes in the in-shore ocean currents, such as are constantly occurring, 

 would remove the consolidated sands from some parts of the reef and 

 heap them in other places. 



Some of the conditions here hypothecated exist at present on the 

 southern coast of California. At and about Oceanside, San Diego 

 County, the region has been depressed and silts have filled up the stream 

 mouths and embay ments. The streams are weak and spasmodic, and 

 unable to keep their mouths open, even when opened by floods. The 

 drainage waters collect in pools and marshes behind the long sand beaches, 

 and escape by percolating through the beach sands. If these sands are 

 calcareous, we may expect in time to see them hardened and turned into 

 long lime-cemented sandstone reefs very like those of northeastern 

 Brazil. 



This hypothesis appears to fit most of the conditions observed along 

 the stone reefs of Brazil : the streams and embayments across which the 

 reefs lie are streams and areas of weak drainage ; the I'eefs have the forms 

 and structure of beaches ; unconsolidated sands underlie the hard reef 

 cap-rock. 



At some places, however, the stone reefs rest against rocks through 

 and from which these lithifying waters could not have passed. At Cape 

 Santo Agostinho, for example, the northern end of the rock reef laps 

 back against granites for a long way north of the embayment behind the 

 Barra do Suape. The percolation of acid waters from marshes and ponds 

 landwai'd of the present reef might have hardened shore sands in front 

 of these marshes into a rock reef, but the sands that lay against the east 

 foce of the cape had no pools to the landward. If they were hardened 

 by this process, the acid w\aters of the marshes must have saturated the 

 sands for considerable distance up and down the coast, as well as in 

 front of the marshes. 



The lianlening .process is not a continuous one. — If the pi'ocess by 

 which the reef rocks are hardened were a continuous one, we might ex- 

 pect to find an overlapping series of sandstones filling the embayments. 

 This overlapping series does not exist. There are sometimes two or 

 three reefs, one behind the other, but even in these cases the reefs are 

 separated from each other by softer layers of sands. The sections of the 

 Pernambuco reef obtained by Sir John Hawkshaw also show that there 

 are loose sands interbedded with the hard layers. The undermining of 

 the reefs by the surf at many places along the coast shows that this 

 alternation of hard and soft beds is a characteristic feature of the large 

 stone reefs. 



