BRANNER: THE STONE REEFS OF BRAZIL. 271 



tinues the firmer corals gradually fill up these spaces so that the inner 

 and older portions of the reef are comparatively smooth and solid. The 

 inner edge of the reef consists of a perpendicular wall not more than five 

 or six feet deep and usually much less, but the outer edge slopes gradu- 

 ally toward the sea for twenty feet or more, and then plunges directly 

 downward as a sheer wall to a depth of twenty -five or thirty feet. This 

 extreme outer edge at the top of the perpendicular wall is covered by two 

 feet of water at low tide, and it is here that the corals grow in the gi-eat- 

 est profusion along the whole face of the seaward wall and as high up as 

 the tides will allow. 



Sand has been constantly accumulating inside the reef so that the inner 

 bay is not more than ten feet deep at any point. There are two or three 

 isolated portions of the reef in this bay that have been nearly covered up 

 by the sand and contain only dead coral rock. 



The reef is everywhere covered by a solid layer of coralline rock. 

 This coralline follows the corals wherever they grow, extending right up 

 to the fleshy part of every coral head. On the older portions of the 

 reef it adds the upper smooth layer, several inches in thickness, that ce- 

 ments together the loose coral rock and makes it lasting. Its importance 

 as a protection to the reef can hardly be overestimated. 



In some places there are elevated spots on the reef formed wholly of 

 encrusting masses of worm tubes. These exist often in the most exposed 

 positions, and form layers of great durability, although they are not of 

 much importance to the reef because of their limited growth. The ex- 

 treme outer edge of the reef where it is exposed to the full force of the 

 breakers is carpeted by small encrusting anemones that are remarkably 

 tenacious in their hold on the rocks, and must save these portions of the 

 reef from much of the wear and tear of the waves. This part of the reef 

 is greatly eaten away, however, by the common rock-boring sea-urchin, 

 Echinometra subangularis, that is very abundant both here and on the 

 sandstone reefs. 



The general surface of the whole reef is quite even, and is exposed 

 nearly two feet at extreme low tide. Tliis exposed portion is composed 

 almost entirely of coralline rock and various encrusting layers of worm 

 tubes, etc. At several points near the northern and oldest part of the 

 reef, however, there is solid coral rock well above the low-water mark, 

 which seems to show that the oldest portions of the reef at least have 

 once been considerably higher than their present level. One of these 

 elevated portions consists of a solid mass of ^fiUepore rock rising at 

 least eight feet above the general surface of the reef, and is exposed even 



