1886.] Alteration by Heat in certain Vitreous Rocks. 433 



as already stated, numerous microliths, mostly bacillar or spicular, 

 sometimes in rectangular forms, and often shaped like a butcher's 

 meat-tray. In the artificially heated specimen a remarkably vesicular 

 structure has been developed, the rock, in fact, has become filled with 

 vesicles, mostly spherical, or approximately so. The sand in which 

 the specimen was heated has adhered firmly to its surface. Vitreous 

 lustre is visible on fractures. Under the microscope between crossed 

 Nicols the rock shows no sign of devitrification from its protracted 

 heating, but the section is full of the large vesicles which have been 

 developed (fig. 2, Plate 4). The glass still contains great numbers of 

 microliths, a few small stellate or cruciform groups being here and 

 there visible, but it seems probable that they are fresh developments, 

 and that the old ones have been dissolved, for there is no longer any 

 ban dftrl. structure, or only very faint indications of it, the microliths 

 i\m Eft all directions and not in streams. This view is favoured by 

 the almost necessary assumption that the rock must have been 

 reduced to a condition bordering on fusion to have permitted the 

 development of such an extremely vesicular structure, while further 

 evidence of this is seen in the firm adhesion to the surface of the 

 specimen of the sand-grains in which it was embedded during the 

 process of heating. In spite, however, of the great molecular change 

 of position implied by this development of vesicles, there is no sign of 

 devitrification, unless indeed the microliths be fresh ones formed 

 during the heating of the specimen, or during two days, in which it 

 cooled from 800° to about 100° C. They do not probably equal those 

 present in the normal condition of the rock. 



Fig. 4, Plate 4, represents part of a section of a piece of the same 

 obsidian, which was kept for 701 hours at a temperature ranging from 

 about 850° to 1100° C. The specimen has been nearly fused, and is 

 pitted on the surface by the impressions of the sand-grains in which 

 it was embedded, a few of the grains still adhering. There is a 

 resinous or subvitreous lustre on some parts of the specimen, but one 

 face is dull. Internally it is full of vesicles, but a thin compact crust 

 exists in which there are none, and this crust is continuous with the 

 spongy vesicular portion. One or two of the cavities are nearly 

 half an inch in diameter, i.e., they occupy nearly the whole thickness 

 of the specimen, while others are of very small dimensions. They are 

 irregular in form, and appear as a rule to communicate with one 

 another. 



Under the microscope, with an amplification of twenty-five diameters, 

 the section shows large irregular vesicles ; their walls {i.e., what 

 remains of the rock) appearing to consist of greenish-brown matter 

 traversed by opaque and approximately parallel bands. The trans- 

 lucent portions of the section seem, under this low power, to consist 

 of micro-crystalline matter, the general aspect being that of fine dust 



VOL. XL. 2 G 



