434 



Mr. F. Rutley. 



[May 27, 



mixed with a felted microlithic substance, while between crossed 

 Nicols numerous doubly refracting granules and needles are visible. 



Where the actual margin or outer crust of the specimen is included 

 in the section the substance is quite transparent and colourless by 

 ordinary transmitted light, and is seen to contain numerous green 

 microliths. 



By reflected light the whole section appears of a greyish- white, 

 except the parallel bands, which are of a rather darker grey, and 

 the more vitreous portion of the outer crust, which appears dark. 

 The extreme outer crust is seen by substage illumination to be almost 

 or quite opaque. 



Under a power of 250 diameters the outer crust may be distin- 

 guished as consisting of three layers, the outermost of extreme thin- 

 ness, transparent, and coffee-coloured ; the next quite opaque or 

 feebly transmitting a brown or brownish-green light. It is of much 

 greater thickness than the outermost glassy layer, and consists of 

 greenish-brown microliths matted parallel to one another, and 

 directed with their longest axes at an angle to the outer surface of 

 the specimen, the angle being sometimes nearly a right angle, and 

 seldom less than about 20° or 30°. This shades off or fringes off into 

 a clear colourless glass layer, also containing numerous greenish - 

 brown spicular microliths, evidently identical in character with those 

 which, by their massing together, form the nearly opaque band last 

 described. It is not easy to say what these microliths are, but they 

 appear to be some form of amphibole or pyroxene ; they have, as a rule, 

 a somewhat frayed and ragged or fibrous aspect, and it seems occa- 

 sionally that they either belong to the rhombic system, or extinguish 

 at a very small angle with their longest axes. With the exception of 

 the glassy band in the thin outer crust, the remainder of the rock 

 has been completely devitrified, fig. 4, Plate 4. Owing to the porous 

 nature of the specimen it was scarcely possible to prepare a very thin 

 section, but, judging from what can be seen, it consists of doubly re- 

 fracting microliths with an admixture of minute crystalline granules. 



The devitrification does not in this case appear to be precisely of the 

 same character as that met with in naturally devitrified obsidians, but 

 at all events we have here a proof that the action of dry heat during 

 701 hours has been capable of devitrifying this glassy rock. 



The next specimen, a black obsidian from the Yellowstone Dis- 

 trict, Montana, U.S., was in the first instance kept at a temperature 

 ranging from 500° to about 1100° C. for 216 hours. Some of the sand 

 in which the specimen was heated adheres firmly to its surface. On 

 fractures the rock is still vitreous in lustre, but it appears of a much 

 paler colour than in its natural condition. This is probably due to 

 the development of great numbers of small vesicles, the colour being 

 now grey, whereas in the unaltered specimen it was black. 



