branner: the stone reefs of brazil. 



109 



each other. The composition is treated under the head of " Consolida- 

 tion" in Part VI. 



The structure of a stone reef in cross-section is well shown in the 

 accompanying figure, which was first published by Hartt in 1870. 



JFlgTiWaiet* - '^ -"— * 



Fig. 65. Theoretic section across a stone reef. — Hartt. 



The same structure is shown in Wagoner's section of the Porto Seguro 

 reef. (See p. 96.) 



In the undisturbed rocks the dip is almost invariably seaward, and 

 the angles are the low angles common to beach sands. I have often 

 measured the slopes of the beaches, and in a few instances I have been 

 able to measure the dips of the reef rocks. At Piedade, a village on 

 the coast a few kilometres south of Pernambuco, the rocks of the stone 

 reef dip at angles varying between 4^ and 6^ degrees ; the beach sands 

 at the same place dip at angles varying between four and five degrees. 

 At Venda Grande, just south of Piedade, the beach has a slope from 

 four to five degrees. I have measured sand slopes as high as 37°, but 

 these were cases of false bedding. The steepest beach noted on which 

 the waves played was at the mouth of Rio Sapucahy, where it had a 

 slope of 11°. 



At Pedras Pretas, the small cape just north of Cabo Santo Agostinho, 

 a small stone reef has dips varying between five and eight degrees. The 

 beach sands have similar dips. The stone reef of Pratagy shows its 

 seaward dip well, but no angles Avere measured. Some of the photo- 

 graphs of the Mamanguape reef also show the seaward dip of the rocks. 

 Occasionally one sees strongly marked false bedding in the reef rocks. 

 Cases of the kind have been noted on the Mamanguape reef. At Rio 

 Grande do Norte the reef, as seen from the lighthouse on the reef, shows 

 the beach structure fairly well. 



Mention should be made here of the statements of Liais that the reef 

 rocks are sometimes vertical. As pointed out in Part V., no cases are 

 known to the writer in which the reef rocks are vertical or have vcr}' 

 high dips except where certain blocks have undermined and have 

 tipped up on one side. 



