1886.] Alteration by Heat in certain Vitreous Rocks, 439 



and it is hoped that another series of experiments may throw a better 

 light upon the threshold of this subject. 



The pitchstone from Arran (figs. 2, 4, and 6, Plate 3), the obsidian 

 from Ascension (fig. 2, Plate 4), the obsidian from the Yellowstone 

 (fig. 6, Plate 4), and the basalt- glass from Kilauea (fig. 2, Plate 5), 

 were all heated at the same time and under the same conditions as the 

 slab of " British plate " glass represented in figs. 1 and 2, Plate 6, in a 

 paper on devitrified glass laid before this Society last year.* 



This slab was -f inch thick, and the heat employed was sufficient to 

 completely devitrify it, and also to partially fuse the glass. 



Another piece of " rough plate " glass, one inch thick, containing less 

 lime, and consequently less easily devitrified than the slab just 

 mentioned, was also heated iu the same kiln as the obsidians 

 described in this paper, and was completely devitrified, with the 

 exception of a spot about J inch in diameter near the centre. 



The obsidian from Ascension (fig. 4, Plate 4), and that from the 

 Yellowstone (fig. 8 and small central figures on Plate 4) were in the 

 kiln 701 hours. In the same kiln were the following specimens of 

 artificial glass figured in the paper just cited. 



Specimen 132, fig. 4, Plate 6. 

 „ 136, „ 5, ,, 

 ,, 137, „ 3, „ 

 143, „ 9, Plate 4. 



The cause of the vesicular character, and the changes in volume 

 produced by dry heat in the rocks described in this paper, the 

 enormous swelling which is frequently manifested, and, on continuance 

 of heat, the return to about the original bulk, without appreciable 

 loss of weight, and also the question of the presence of water, will 

 supply materials for a future paper, in which it is hoped that some 

 experiments on changes induced in vitreous rocks by heat in presence 

 of water vapour may also be described. 



Alteration of Pitchstone. 

 Plate 3. 



1. Pitchstone, Corriegills, Arran. X 25. 



2. The same kept at a dry heat ranging up to about 1100° C, 216 



hours. X 25. 

 3f. Pitchstone, Corriegills. X 250. 



4. The same after heating from 500° to cir. 1100° C, for 216 hours. 

 X 250. Showing the alteration of the hornblende belonites 

 to shrivelled (rusty coloured) bodies, and the coarse spicula> 

 developed in certain parts of the ground-mass. 



* " Proc. Koy. Soc," vol. 39 (1885), pp. 88-107. 



f Figs. 1, 3, and 5, Plate 3, represent the rock in its normal condition. 



