SINGULAR REEF 529 



The most curious object which I saw in this neighbour- 

 hood was the reef that forms the harbour. I doubt whether in 

 the whole world any other natural structure has so artificial an 

 appearance.^ It runs for a length of several miles in an 

 absolutely straight line, parallel to and not far distant from 

 the shore. It varies in width from thirty to sixty yards, and 

 its surface is level and smooth ; it is composed of obscurely- 

 stratified hard sandstone. At high water the waves break 

 over it ; at low water its summit is left dry, and it might then 

 be mistaken for a breakwater erected by Cyclopean workmen. 

 On this coast the currents of the sea tend to throw up in front 

 of the land long spits and bars of loose sand, and on one of 

 these part of the town of Pernambuco stands. In former 

 times a long spit of this nature seems to have become con- 



CICADA HOMOPTERA. 



solidated by the percolation of calcareous matter, and after- 

 wards to have been gradually upheaved ; the outer and loose 

 parts during this process having been worn away by the action 

 of the sea, and the solid nucleus left as we now see it. 

 Although night and day the waves of the open Atlantic, turbid 

 with sediment, are driven against the steep outside edges of 

 this wall of stone, yet the oldest pilots know of no tradition of 

 any change in its appearance. This durability is much the 

 most curious fact in its history ; it is due to a tough layer, a 

 few inches thick, of calcareous matter, wholly formed by the 

 successive growth and death of the small shells of Serpulae, 

 together with some few barnacles and nulliporae. These 

 nulliporae, which are hard, very simply- organised sea-plants, 

 play an analogous and important part in protecting the upper 

 surfaces of coral-reefs, behind and within the breakers, where 



^ I have described this Bar in detail in the Land, ami Edin. Phil. ^dag. vol, 

 xix. (1841), p. 257. 



35 



