46 



A NATURALIST ON THE "CHALLENGER. 



in the altitudes at which plants can grow at various aspects in 

 St. Vincent. Thus Aizoon canariense, a Malvaceous plant which 

 on Bird Eock grows close to the sea level on its windward side, 

 does not commence on the leeward side of the hills of the main 

 island till 700 or 800 feet. On the mountains on the southern 

 side of the island, the vegetation does not come so far down 

 the windward slopes, since the wind is heated and dried before 

 reaching them, by passing over the hot central plain. 



Vertical dikes of basalt are very numerous all over the island, 

 penetrating the main component rocks, by the disintegration of 

 which they are often weathered out so as to project as walls. 

 They usually show a columnar structure, the columns being as 

 usual at right angles to the cooling surfaces. I saw several in 

 which the cleavage in the centres of the masses was laminar 

 and parallel to the lateral surfaces, whilst on either side the dikes 

 were composed of very regular small columns disposed at right 

 angles to these surfaces. In the Auvergne 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. 





^Oif[l&> 



sit] 



so 



io 





3) 



a 



DIAGRAMS OF THE CLEAVAGE STRUCTURE OF BASALTIC DYKES. 



1 In S. Vincente; 2 in the Auvergne, near M. Dore; a central portion with laminar cleavage ; 

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

 d central mass with horizontal columnar cleavage. 



On Bird Island, the rocks about tide mark, are covered 

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

 nacese, 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 Corallinaceae are seaweeds which secrete a 

 dense skeleton of carbonate of lime. The incrustation on Bird 



