Upper 

 Gallery. 



Wall-case 2. 



156 



VOLCANIC ROCKS OF ASCENSION 



This rock was ejected amongst cinders from one of the more 



recent volcanos. 



(See Darwin " On Volcanic Islands," p. 41.) 



92. — Ejected fragment, greenstone, composed of crystals 



of Labrador Felspar •, a little altered hornblende, and scales 



anular f\ 



© 



stices. 

 (Se< 



9° 



99 



p. 41.) 



■Ejected fragment, portions of hard siliceous 

 plate-like veins, of varying thickness, intersecting the 

 earthy trachytic masses on the flanks of the " crater of 



the old volcano." 



(See Darwin " On Volcanic Islands," pp. 44 and 45.) 



94. — Seams of compact oxide of iron, occurring con- 

 formably in the lower parts of a stratified mass of ashes 

 and fragments. 



" This seam of compact stone, by intercepting the little 

 rainwater which falls on the island, gives rise to a small 

 dripping spring, first discovered by Dampier. It is the only 

 fresh water on the island, so that the possibility of its being 

 inhabited has entirely depended on the occurrence of this 

 ferruginous layer." — Darwin " On Volcanic Islands," p. 39. 



in altered 



occurring 



95. — White siliceous sinter, 



trachyte. 



(See Darwin " On Volcanic Islands," p. 45.) 



96. Seams of siliceous sinter, occurring in altered 



trachyte. 



97. — White siliceous sinter. 



98. — Cream-coloured siliceous sinter. 



99. Siliceous sinter, formed of thin irregular plates 



of chalcedonic quartz, occurring in altered trachyte. 



100. — Ochreous-brown coloured jasper, occurring in large 

 irregular masses, and sometimes in veins, both in altered 

 trachyte, and in an associated mass of scoriaceous basalt. 



(See Darwin " On Volcanic Islands," p. 46. 



101. — Another variety, 



inclosing 



irregular 



angular 



patches of red jasper, with their edges blending into the 

 surrounding mass. 





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