40 CAPE VERDE. 



encountered as the mountains are ascended, and at i,ooo feet 

 most of it ceases. 



On some active volcanoes, however, as at the Banda Group 

 near the Moluccas, a gradual decrease in the vegetation in 

 correspondence with increased altitude is brought about by 

 exactly opposite conditions, namely, gradual increase of heat. 

 Here, close to the crater at the summit^ the soil is excessively 

 hot, yet one or two plants grow in it where it is almost too hot 

 for the botanist's hand, and these straggle upwards, beyond, 

 distancing more sensitive competitors, till a region is reached 

 which is barren of all but lowly organized alga3, which grow 

 around the mouths of natural steam jets, as about the hot 

 springs in the Azores and elsewhere. 



In very high latitudes only, apparently, is the vegetation not 

 influenced by altitude. On the mountains in East Greenland, 

 the same plants extend from sea level up to as high as 7,000 

 feet altitude. This circumstance is accounted for by the fact 

 that here the sun never rising far above the horizon, its rays 

 strike the mountain-slopes nearly or quite vertically, and hence 

 by their greater power compensate for the larger amount of 

 heat lost by radiation at great elevations. The flat land receives 

 the rays on the other hand very obliquely, and hence with 

 much less force.* 



The combination of effects due to difference of aspect with 

 regard to the trade wind and sun produces a marked difference 

 in the altitudes at which plants can grow at various aspects 

 in St. Vincent. Thus Aizoon cajiariejise, a Malvaceous plant 

 which on Bird Rock grows close to the sea level on its wind- 

 ward 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 disin- 

 tegration 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 



* "Die Zweite Deutsche Nordpolarfahrt in den Jahren 1869 und 1870," 

 2ter Bd. Wissenschaftliche Ergebnisse. Leipzig, F. A. Brockhaus. 

 "Klima und Pflanzenleben auf Ostgronland,'' von Adolph Pansch in Kiel. 



