Chap, ii.] 



CALCAREOUS SEAWEEDS. 



41 



district, I have observed dikes in which laminar cleavage 

 parallel to the surfaces occurred at the sides of the dikes and 

 the columnar cleavage in the centre, just the opposite condition. 

 On Bird Island, the rocks about tide mark are covered 

 with a broad band of a dense incrustation composed of Coralli- 

 naces, which forms a striking feature in the appearance of the 

 island as seen from the sea, and is more marked here than on 

 the main island. The Corallinaceje are seaweeds which secrete 

 a dense skeleton of carbonate of lime. The incrustation on 

 Bird Island is of several colours, white, bright pink or cream 

 colour, and is mainly composed of two species, of Lithotham- 

 nio?t, L. polymorphiun and L. viamillare. The incrustation 

 assumes very varied forms, being simply incrusting, and follow- 

 ing the form of the rock surface on which it rests, or forming 

 smooth rounded convex masses, or being covered with a close 





DIAGRAMS OF THE CLEAVAGE STRUCTURE OF BASALTIC DYKES. 



I In St. Vincent; 2 in the Auvergne, near M. Dora; a central portion with 

 laminar cleavage; h lateral regions with horizontal columnar cleavage; c lateral 

 regions with laminar cleavage ; d central mass with horizontal columnar cleavage. 



set series of projections, sometimes of considerable length, and 

 with a sinuous arrangement. 



I broke off specimens from the mass with my geological 

 hammer. It is bored in all directions by MoUusks, such as 

 Lithodomus caudigerus — a Senegambian species with two curious 

 little tails at the hinder extremities of the valves so cut out as 

 to lap over one another when the shells are closed. On the 

 whole, plant-life seems to play a far more important role than 

 do corals in accumulating carbonate of lime around the Cape 

 Verdes. The principal role in this respect is however played 

 by the larger Foraminifera, of the shells of which the calcareous 

 sand of St. Vincent is mainly composed. 



I made excursions every day along the shore or over the hot 

 sandy plains or over the sharp and rugged lava, in search of 

 plants and animals. So desolate is the place that a naval 



