1894-95.] 22 5 



I believe, then, that the striking resemblance of typical 

 "hullite" to lumps of tachylyte (or ''pitchstone" and "obsidian" 

 of older authors) is by no means accidental ; the material is a 

 basic glass that has become soft and " gummy " by alteration. 

 Mr. Hardman's* comments on those who had " mistaken " it 

 for a glassy substance may, I think, be set aside ; but it must be 

 remembered that, when he wrote, palagonite and tachylyte had 

 been little studied in our islands. 



In two sections from Carnmoney, which are in the collection 

 of the Royal College of Science for Ireland, the brown u hullite" 

 can be seen entering the corroded felspar crystals and playing 

 precisely the part of an original glassy groundmass. Comparison 

 is possible, moreover, with the paler and unaltered glass which 

 occurs interstitially in some parts of one of the sections. The 

 black groups of crystallites of magnetite in the glass are not, 

 however, present in the altered product, and probably the warm 

 brown colour of sections of "hullite" is original, owing to the 

 presence of widely diffused ferric compounds. Where separation 

 of the materials occurred, as in the paler areas, the glass became 

 more stable ; and the soft palagonitic condition arose, as usual, 

 most completely in the undifferentiated portions. 



The black material lining the steam-cavities of the Carn- 

 money basalt certainly seems the same as that which forms the 

 tachylytic blebs throughout the rock. But its stalactitic and 

 encrusting form in no way prevents us from regarding it as also 

 originally a glass. On a minute scale, it forms a reproduction 

 of the lava-stalactites in the caverns of the Hawaiian flows ; the 

 glassy matrix of the lava has oozed out under pressure into any 

 cavities it could find. Mr. Teallt has already shown how 

 amygdaloids of this kind may be produced, in which the steam- 

 vesicles have become converted into spherical lumps of glass, 

 looking curiously at variance with the far more crystalline 

 material of the andesitic or basaltic mesh work round them. 



* Work quoted, p. 161. 



f "On the Amygdaloids of the Tynemouth Dyke," Geol. Mag., 1889, p. 481. 



