110 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Considered in relation to underlying rocks, the structure of the stone 

 reefs is best understood from the results obtained by the holes bored in 

 the Pernambuco reef in 1874 by Sir John Hawkshaw. The following is 

 the record of the deepest hole ^ put down. 



Kecokd of Boring on the Pernambuco Reef. 



Metres. 



Reef rock, hard 2.95 



White sand 1.22 



Shells 1.10 



Gray sand 0.65 



Broken rock 1.27 



Dark sand 2.10 



Mottled clay 1.80 



Yellow clay 0.70 



Gray sand 3.00 



White sand 2.20 



Other holes put down in the reef show that the surface rock is underlain 

 by sands, shells, and clays, but there is no regularity in the sequence of 

 these materials. 



Conclusions. — The stone reefs are nearly, but not quite, straight. 

 The bedding of the materials dips seaward at the same angle as ordinary 

 beach sands. The hard rock of the reef is only three or four metres 

 thick. The underlying materials are sands, shells, and clays ■without 

 regular sequence. The process of formation, the character, and the 

 structure of the reefs show that they are ancient beaches hardened by 

 lime carbonate, while their straightness shows that they are forms of a 

 mature beach-line fixed and made permanent by the process of consoli- 

 dation pointed out in Part VI., pp. 171-198. 



1 Melhoramento dos portos do Brazil, p. 15. Rio de Janeiro, 1875. 



