440 



Alteration by Heat in certain Vitreous Hocks. [May 27, 



5*. Pitchstone, Corriegills. X 1150. Showing portion of the 

 ground-mass containing globulites and microliths. 



6. The same after heating from 500° to cir. 1100° C. for 216 

 hours. X 1150. Showing the alteration which the ground-mass, 

 corresponding to that in fig. 5, has undergone. 



Alteration of Obsidian. 

 Plate 4. 



If. Black obsidian, Ascension. X 25. 



2. The same after heating for 216 hours from 500° to 1100° C. 

 x25. 



3f. Black obsidian, Ascension. X570. 



4. The same after heating for 70 L hours from 850° to 1100 C. X 570. 

 5f. Black obsidian, Yellowstone, Montana, U.S. x25. 

 6. The same after heating for 216 hours from 500° to 1100 C. x 25. 

 7f. Black obsidian, Yellowstone. X 570. 



8. The same after heating for 701 hours from 850° to 1100 C. X 570. 



Of the small figures occupying the middle line in the plate the 

 three upper ones represent greyish- white crystalline pellets, 

 frequently hollow, and often bearing minute crystals of 

 specular iron. These pellets were taken from the large 

 vesicles developed in the Yellowstone obsidian by heating from 

 850° to 1100° C. during 701 hours, x 4. 



The lowest figure in the middle line represents one of the minute 

 crystals of specular iron which was attached to the oater 

 surface of one of the above pellets. X 120. 



Alteration of Basalt-glass. 

 Plate 5. 



lj. Vesicular basalt-glass, Kilauea, Sandwich Islands, x 25. 



2. The same after exposure to a dry heat for 260 hours at a tem- 



perature ranging from about 700° to 1200° C. x 25. 



3. Vesicular glass formed by the fusion of basalt from the Giant's 



Causeway, Antrim, in a Stourbridge-clay crucible ; the fused 

 mass having been rapidly cooled, x 55. 



4. The same rock similarly fused and slowly cooled. X 55. 



5. Fragment of basalt (Giant's Causeway) placed on the surface of 



a molten mass, similar to No. 3, and allowed to sink into it. 

 On the left is the fragment of basalt, and on the right the 



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

 f Figs. 1, 3, 5, and 7, Plate 4, represent the rocks in their normal condition. 

 X Fig. 1 and the left half of fig. 5, Plate 5, represent the rocks in their normal 

 conditions. In the latter case, however, there may be slight alteration. 



