224 [Pwe. B.N.F.C., 



basalt. He notices that certain fibres affect polarised light, in 

 the midst of the generally isotropic material, and concludes that 

 " hullite " is related to " the gummy products to which the 

 decomposition of olivine gives rise." 



But this in no way gets over the difficulty of its being again 

 and again an interstitial substance between the rods of felspar 

 and the granules of augite. While it coats the interior of 

 vesicles on the one hand, on the other it plays the part of a true 

 groundwork to the crystalline constituents. 



This latter fact is so clear that I have no hesitation in regard- 

 ing the great mass of what has been styled " hullite" as simply 

 the altered and hydrated glass of the original basaltic ground- 

 mass. Its chemical composition is closely paralleled by the 

 frequently analysed " palagonite," which itself has no longer 

 any position as an independent mineral species. Prof. A. Geikie* 

 long ago showed how fragments of basaltic glass had been 

 altered to green " serpentinous " material in the tuffs of Scot- 

 land ; while " palagonite " has been also described from the 

 relics of Ordovician volcanoes in Wales.t In such cases the 

 old basaltic glass or tachylyte is altered to a dull soft black 

 substance, while retaining in a wonderful degree its former 

 structures and its relation to the minerals which once developed 

 from it, or which floated in it porphyritically. Under the 

 microscope it is difficult to believe that we are dealing with a 

 substance so highly altered as palagonite ; but the polariscope, 

 as, indeed, in some parts of " hullite," often reveals a tufted 

 crystalline structure, which has arisen during the hydration. 

 IX have commented on this point recently in the case of a 

 material which is intermediate between tachylyte and "hullite," 

 and which has a hardness, as I now notice, of 4*5. 



• " On the Carboniferous Volcanic Rocks of the Basin of the Firth of Forth," 

 Trans. Roy. Soc, Edin., vol. XXIX (1879), P S l S and P 1 - xn - 



t G. A. J. Cole, " On some additional occurrences of Tachylyte," Quart. Journ. 

 Geol. Soc, London, vol. XLIV (1888), p. 306. 



X " On Variolite and other Tachylytes at Dunmore Head, Co. Down," Geol. Mag., 

 1894, p. 222. 



