464 PKOF. «T. W. JTJDD AND G. A. J. COLE ON THE 



Fig. 5. In this rock (the basalt- glass of Gribun, in Mull) we find that, in 

 addition to the perfect separation of magnetite in skeleton 

 crystals, the formation of transparent crystallites (belonites) has 

 gone on to a considerable extent. 



Fig. 6. A glassy lava from Hawaii, consisting of a clear brown glass with 

 a little cloudy material in parts, and a few crystallites. Some of 

 these latter resemble the central portions of the structures to 

 which Krukenberg gave the name of " chiasmoliths." 



Discussion. 



Professor Bonnet expressed his sense of the value of the paper. 

 Students of British petrology would be much indebted to the authors 

 for this exhaustive notice of so interesting and rare a rock. As a 

 proof of its rarity, he said that he had examined numerous basalt 

 dykes in Scotland and elsewhere before he found a satisfactory spe- 

 cimen of tachylyte ; and that, by a fortunate accident, appeared to be 

 in another locality. It was by the road to Goatfell from Brodick, 

 in Arran, at the back of the outbuildings of the Castle. Two veins 

 of compact basalt, about 1 foot thick, intrusive in an older decom- 

 posed basalt, had an edging of tachylyte from J inch to 1 inch thick. 

 As regards the Sandwich-Island lavas, he believed that the masses 

 of tachylyte were especially connected with Kilauea (for ordinary 

 basalts were common in the islands) ; might this be due to the ex- 

 ceptional condition of the lava in the crater of Kilauea, such an 

 enormous mass of molten material? 



Mr. Teall stated that the comparative rarity of basalt- glass in 

 nature appeared somewhat remarkable when the readiness with 

 which basalt can be fused and made to assume the condition of 

 glass by rapid cooling is taken into consideration. He suggested 

 that the apparent anomaly might be due to the lower fusibility of 

 the basic as compared with the acidic material. The lower fusibility 

 necessarily involved a greater prolongation of the time during which 

 the conditions were favourable to crystal-development. The forma- 

 tion of basalt-glass on the large scale in Hawaii might be due to 

 the fact that the material before being ejected is cooled throughout 

 its mass by convection-currents almost to the point of consolidation 

 before being erupted as lava. According to this view the average 

 temperature of emission of the glassy lavas of Hawaii would be less 

 than that of a lava cooling to normal basalt. 



"With regard to the crystals referred to by the author he thought 

 that the honeycomb-structure might in part, at any rate, be due to 

 the mode of crystal-growth. 



Mr. Batjekhan expressed the satisfaction which the paper had 

 given him. The analysis of the Beal rock indicated a remarkably 

 large proportion of alkalies. He had recently investigated a case 

 of the formation of crystalline silicates and aluminates in a slag by 

 the action of blast-furnace gas containing alkalies and oxides of 

 zinc and manganese upon the firebricks of a hot-blast stove. The 

 high density of the basalt-glass was probably due to separated 

 magnetic oxide of iron. The structure, when seen under a high 



