74 Ascension. 



PAET I. 



posed with nodular obsidian, it would only be necessary, 

 in accordance with, the foregoiug facts, to suppose that 

 lava in these instances had been erupted with its ingre- 

 dients mixed in the same proportion, as in the concre- 

 tionary obsidian. 



Lamination of volcanic rods of the tracliytic series. 



We have seen that, in several and widely distant 

 countries, the strata alternating with beds of obsidian, 

 are highly laminated. The nodules, also, both large 

 and small, of the obsidian, are zoned with different 

 shades of colour ; and I have seen a specimen from 

 Mexico in Mr. Stokes' collection, with its external 

 surface weathered 1 into ridges and furrows, correspond- 

 ing with the zones of different degrees of glassiness : 

 Humboldt, 2 moreover, found on the Peak of Teneriffe, 

 a stream of obsidian divided by very thin, alternating, 

 layers of pumice. Many other lavas of the feld spathic 

 series are laminated ; thus, masses of common trachyte 

 at Ascension are divided by fine earthy lines, along 

 which the rock splits, separating thin layers of slightly 

 different shades of colour ; the greater number, also, of 

 the embedded crystals of glassy feldspar are placed 

 lengthways in the same direction. Mr. P. Scrope 3 has 

 described a remarkable columnar trachyte in the 

 Panza Islands, which seems to have been injected into 

 an overlying mass of tracliytic conglomerate : it is 

 striped with zones, often of extreme tenuity, of dif- 

 ferent textures and colours; the harder and darker 



1 MacCulloch states (' Classification of Rocks,' p. 531), that the 

 exposed surfaces of the pitchstone dikes in Arran are furrowed, 

 ' with undulating lines, resembling certain varieties of marbled 

 paper, and which evidently result from some corresponding difference 

 of laminar structure.' 



2 ' Personal Narrative,' vol. i. p. 222. 



3 ' Geological Transactions,' vol. ii. (second series) p. 195. 



