1905.] 



CAPE VERDE MARINE FAUNA. 



183 



but in one part a lai-ger channel exists on its land side. In this 

 a constant current is produced by the flowing away of the spray 

 thi-own over the reef -edge to the northerly opening of the channel. 

 Obviously this current is the means by which the channel was 

 produced. Landwards is a sand- beach, and in places a small 

 amount of very soft beach-sandstone, at a level of about three 

 feet vertically above the i-eef-flat. 



Text-fig. 26. 



Chart of Porto Grande. 



The edge is irregular, consisting of a line of rock-masses of 

 nearly equal height, i. e. a few inches higher than the general 

 level, and usually perfectly continuous with the reef-flat, and its 

 surface is completely covered with nullipore giving a characteristic 

 irregularity. There is a precipitous drop into one or two fathoms 

 of water, beyond which is a slope of sand a,nd stones. In short, 

 all appearances are those of a growing nullipore-reef with beach- 

 sandstone towards its landward side, but examination shows 

 that sandstone extends to the seaward edge, being merely plastered 



