BASALT-GLASS OP THE WESTERN ISLES OF SCOTLAND. 465 



magnifying power, was analogous to that of certain iron-furnace slags, 

 which also vitrified at the edges. He thought the imperfections of 

 the crystals due to enclosures of glass in their formation rather than 

 to subsequent corrosion. 



Mr. W. Mtjie spoke of the rarity of tachylyte, and said that the 

 dykes could not have run for a long time ; if the glass occurred, it 

 would be a sign of rapid cooling. 



Prof. Seelet asked how those basalt-glasses of Scotland differed 

 from those of the basalts of Europe which differed chemically and 

 mineralogically from them. The Sandwich-island glasses were very 

 rich in iron. Might the greater abundance of the glass be attri- 

 buted to the iron or to the rate of motion ? He called attention to 

 the fact that the basalt-glass of the Sandwich Islands adhered to the 

 branches of trees without scorching them. 



Mr. Koch said that he had found a tachylyte in Skye, and near 

 the leaf-beds in Mull; it occurred in bean-shaped masses and 

 strings, generally of great thinness. He had made experiments in 

 Siemens's furnaces. He found that slag was partly affected by the 

 amount of moisture contained : and this was confirmed by experi- 

 ments with slag. If run into moist sand, the lower part of the slag 

 was glassy. He therefore attributed the glassy condition to the 

 action of steam. If one slag was poured on another, a glass would 

 be produced at the point of junction. At the Sandwich Islands he 

 thought that the basalt was at a very high temperature. The 

 opaque glass was formed at the lower temperature in experiments. 

 In experiments with crystals dropped into molten slag the edges 

 were fused. 



Prof. Judd said that similar glassy lavas flowed from Mauna 

 Loa and Kilauea ; so there did not seem to be any special reason for 

 assigning the glassy state of the lavas to the peculiar conditions at 

 the latter crater. He thought it more probable that the forms in 

 the crystals were due to corrosion than to inclusion. A high per- 

 centage of alkali was common to several of the basalt-glasses 

 of Germany. The question of the composition of the basalt was 

 discussed in the paper. Cohen, however, had proved that there 

 were glassy lavas identical in composition with true basalts. The 

 rock Mr. Koch had found had not been proved to be a true basalt- 

 glass. The quantity of moisture varied much in analysis, and truly 

 vitreous rocks, like obsidian, seemed to be less rich in water than 

 subvitreous ones, such as pitchstones. 



