

AND ASCENSION. 



147 



■ 



i 



The greater number of these are composed of pearlstone 

 and pitchstone, with occasional nodules of obsidian, alter- 

 nating with felspathic layers. 



18, 22, and 23 are sphaerulitic, while 16, 19 to 2], 24 and 

 25 contain included crystals of glassy felspar, lying length- 

 ways, or with their longest axes parallel with the lamina in 

 which they are included ; an arrangement due to the motion 

 of the mass while in a heated state. 



27 to 31 show the passage from pitchstone into obsidian. 



It may be observed that pitchstone and obsidian 

 merely different forms of the same substance, caused by the 

 unequal rate at which the liquified mass of melted rock 



are 



has been cooled, 

 volcanic i 



( 



form of 



glass) is from the superficial portion of the 

 mass which has cooled most rapidly, while the more dull, 

 opaque interior portion into which true obsidian passes 

 at a slight depth, and which has cooled more slowly, is 

 termed pitchstone. 



31 to 31c are varieties of volcanic slag, exhibiting dif- 

 ferent degrees of fusion. 



32 to 62 are from the series of trachytic rocks which form 

 the more elevated and central, and likewise the south-east 

 part of the island. 57 to 58 are augitic lavas, with included 

 crystals of glassy felspar. 



63 and 64 are pumiceous. 65 to 67 volcanic ashes and 

 sand, or the more finely divided products of eruption. 



68 to 73 are varieties of softer tufa, and 105 are concre- 

 tions which occasionally occur in it. 



85 to 92 are volcanic bombs and fragments of rocks which 

 have been shot forth during aeriform explosions, and are now 

 found mixed with masses of scoriae. 86 to 88 exhibit strik- 

 ing proofs of their having been in a fluid state, and of 

 having had a rotatory motion communicated to them when 

 originally vomited from the crater. This is especially to 

 be observed in 86. 



Siliceous sinter is represented by 95 to 99. It occurs in 

 the altered trachytes either in the form of irregular masses 



V 



Gallery. 



Wall-case 2. 



