76 



THE VOYAGE OF H.M.S. CHALLENGER. 



Torrents and atmospheric erosion have worn gullies in these walls of rock, and heaped 

 together piles of debris, which have accumulated to a height of 100 feet at the foot 

 of the cliffs. This circle of volcanic fragments is, in its turn, edged by a belt of gravel 

 of the same nature, which is spread out on the narrow shore of the island. 



There is perhaps no region in the world where atmospheric agencies exert their 

 destructive action in so energetic a manner as here. For nine months in the year 

 terrible tempests run riot on the island, and when the season of rains has ended, and 

 the snow that has accumulated on the top of the peak begins to melt, the water rushes 



The Island of Tristan da Cunha. 



down in cascades, carrying an immense quantity of debris. These streams vigorously 

 attack and demolish the less coherent and homogeneous of the layers that form the 

 horizontal strata ; they lay bare the rocks of the dykes, and cut deep indentations in 

 the ledge of the terrace. The transverse dykes alone resist the erosion, and stand up 

 like walls. 



Mr. Buchanan observes that at Tristan, as at Nightingale Island, the dykes 

 have, at their contact, made the volcanic breccia which they traverse more alterable ; 

 whence it results that denudation acts by preference along their sides. These dykes 

 of massive injected rocks also form the axis along which the coves and bends of the 



