branner: the stone reefs of brazil. 



77 



Many of the minor pools on top of the reef contain calcareous growths 

 and seaweeds in cavities built up by Serpulae, etc. 



Sometimes these calcareous growths fasten themselves on points of 

 sandstone. Here and there sea-urchins with their burrows cover con- 

 siderable areas, especially on the seaward faces of the reef where the 



Fig. 46. Points on surface of sandstone reef covered with Serpulae and seaweed. 





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waves constantly renew the water. These burrows are for the most part 

 parallel with the currents that flow across them, and hence mostly at 

 right angles to the trend of the reef. The following is the plan of a 

 few of these burrows. 



These pits are only partially occupied by sea-urchins. They contain 

 also corallines and some corals {Pontes). They are cut in the hard 

 surface sandstone, and are from 

 three to ten centimetres deep. The 

 edges of these pits are generally 

 overgrown with Serpulae. In the 

 pools of the reefs surface are occa- 

 sionally found water-worn lumps of 

 Serpulae tubes, apparently broken 

 from the reef and rolled by the 

 waves. 



There are some cracks in the surface of the reef, most of them run- 

 ning lengthwise of it. The cracks become more abundant and more 

 varied where the gaps occur, and they continue so clear across the frac- 

 tured area. In other portions of the reef they are less abundant than 

 on the other stone reefs exaiuined. 



The rock of this reef is, on the whole, finer of gi-ain than that of the 

 average. It contains some pebbles, some of them black iron-colored 

 ones washed from the mottled Tertiary plateaus. Most of the rock is 

 hard, light, sugar-brown sandstone cemented with lime carbonate. 



Everywhere the body of the reef is covered with Algae ; on top they 

 are mostly green. It is etched a little on the landward side, probably 

 by the acid waters of the streams, just enough to bring out the bedding, 



Fig. 47. Plan of the pits on the sur- 

 face of Cabo Santo Agostinho reef. 



