128 



VOLCANIC ROCKS OF ADEN, 



Upper 

 Gallery. 



Wall-case 1 



Wall-case l. 



Arranged and described by H. W. Bristow, F.G.S. 



Shelf 2 contains specimens of the volcanic rocks of the 



extinct volcano of Aden , numbered 1 to 19. 



Nos 



peninsula, and the passage from compact basalt into porous 

 cellular lava ; they also serve to exemplify the manner in 

 which such lavas, after cooling, become coated and partly 



Quartz 



8 and 9 are 



varieties of the less fusible felspathic lava, Trachyte. 10 to 



* 



13 are the more intensely fused lavas, which by a more or 

 less rapid rate of cooling pass from a real transparent vol- 



(12) 



stance Pitchstone. 



(14 



the interior of the crater like an ordinary lava-stream, but 

 has been ejected during the periodical explosions which 

 take place simultaneously with the more quiet action of the 

 volcano, when ashes, cinders, and fragments and masses of 

 rock, and melted lava are hurled into the air, frequently to 

 enormous heights, by the influence of pent-up steam and 



, Of the substances so ejected certain portions fall 

 back again into the interior, or accumulating around its 

 mouth serve to increase the height of the already existing 

 crater, while other portions, being scattered far and wide, 

 cover the surface of the country for many miles. 



Pumice, one of the ejected substances in question, be- 

 coming cooled in its passage through the air, retains the 

 porous, spongy structure it originally possessed, owing to 

 the presence of the vapours or gases with which it was 



permeated. 



" The ashes, cinders, and molten rock ejected, may often 

 be considered as little else than modifications of the same 

 substance," at one time in a state of fusion, at another 

 driven off by vapours and gases in portions of different 



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