148 



VOCLANIC ROCKS 



Upper 

 Gallery. 



Wall-case 2 



or in seams. 



The formation of these, as well as of the thii 

 plate-like veins 93, has been produced by the segregation 



or infiltration of siliceous matter. 



(100 



forms large irregular masses in the altered trachyte, were 

 probably produced by a process (as suggested by Mr! Dar- 

 win) analogous to that by which wood becomes gradually 

 silicified ; that is, by the gradual removal, particle by par- 

 ticle, of the original rock, (in this case a basaltic rock,) ac- 

 companied by the simultaneous substitution of siliceous 

 matter and iron. 



106 to 109 are varieties of scoriaceous lava from Chatham 

 Island; while 110 to 113 are cellular basaltic lavas, the 

 latter containing crystals of olivine. 



116 to 117 are from the Cape de Verd Islands. 



Various kinds of lava from New Zealand (White Island) 

 are represented by Nos. 121 to 126. 127 and 128 are spe- 



129 and 130, gypsum; 131 

 and 132, native sulphur ; and 133, the ashes with which the 

 country around Auckland is covered. 



134 and 135 are from volcanic springs at Kertch, in the 



cimc-ns of siliceous sinter. 



Crimea. 



from the Island of 



Presented by Mr. Charles Darwin, F.R.S., and Captain Ord, 



B.E. 



The Island of Ascension, situated between the coasts of 

 Africa and Brazil, is nine miles long by six in breadth. Its 

 entire surface, which is broken into mountains, hills, and 

 ravines, is covered with ashes, cinders, pumice, and lava. 

 Its general appearance is that of a mass of smooth, bright 

 red conical hills, with truncated summits, rising from a 

 plain of black, sterile lava. The highest point on the island, 

 Green Hill, is 2,870 feet above the sea level.— H. W. B. 



1 & 2. — Volcanic slag, or cinder. 

 3. — Red scoriaceous lava, partly vesicular, from the 

 outer portion of the stream. 



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