brannek: the stone reefs of brazil. 



53 



tions are invariably of very hard rock that rings under the hammer ; 

 they make walking over the surface of the reef in certain places almost 

 impossible. Other points, rounded in outline, but standing at lower 

 levels, are covered with barnacles. 



There are but few places over the surface of the entire reef that have 

 the rock freshly exposed. On the outer face and wherever kept con- 

 stantly moistened with salt water, corallines and other Algae grow in 

 greater or less abundance. Barnacles abound in the gaps and depres- 



FiG. 25. Tide-pool formed by organisms 

 on reef. 



sions in those places that do not receive 

 the full force of the surf and yet are low 

 enough or so situated as not to be long 

 out of the Avater. Young barnacles speck 

 the reef everywhere. The seaward edge, 

 where the reef is flat, generally or always 

 has a little low rim rising as a dam on 

 its margin and enclosing shallow pools of 

 water on top of the reef. 



Oysters are found in patches along the 

 inner face of the reef, especially where 

 the ebbing tide brings the river water 



against it. There are occasional patches of dead oyster shells clinging 

 to this inner part of the reef. 



A large part of the outer surface and much of the top of the reef 

 where it is kept constantly wet by the surf is full of holes and channels 

 occupied by sea-urchins. These holes are two or three inches deep and 

 of various lengths, from a few centimetres to two or three metres. 

 They show a decided tendency to lie parallel to each other and with the 

 direction of the waters that wash over them. In section they are under- 

 cut, as shown in the profile herewith. 



These trenches are not full of sea-urchins, but have a few individuals 

 scattered through them, or, at most, are half full. 



Fig. 26. Sea-urchin burrows 

 on the surface of a sand- 

 stone reef ; plan and sec- 

 tion. 



