436 



Mr. F. Rutley. 



[May 27, 



times lined with crystals apparently of the same mineral. The pellets 

 themselves are too friable to admit of any sections of them being cut, 

 while no satisfactory conclusion has yet been arrived at by crushing 

 them and examining the fragments under the microscope. Small glisten- 

 ing faces sometimes showing a certain parallelism of growth may be 

 detected with the help of a lens, and, so far as general appearances go, 

 the mineral bears a somewhat close resemblance to rhyacolite. They 

 are, at all events, probably anhydrous silicates allied to the felspar or 

 nepheline groups. In some cases the pellets adhere slightly to the 

 walls of the vesicles, yet in one or two instances they appeared to be 

 loose, but may possibly have been detached by the shock in breaking 

 open the specimen. On examining one of these pellets by reflected light 

 under a half-inch objective, the white crystalline surface was seen to 

 be studded with minute black or deep blackish-red crystals, having a 

 brilliant metallic lustre. One of them exhibited a six-sided face as 

 shown in the bottom figure of the middle line in Plate 4. This 

 was turned sufficiently well into position to enable all parts of the face 

 to be brought at once into focus, when it was found that measure- 

 ment of the angle formed by adjacent edges was 60°. There is, 

 therefore, little doubt that these small crystals are specular iron, 

 which has separated out during the process of artificial heating, no 

 such crystals being visible in a microscopic section of the rock in its 

 normal condition. 



Under a power of 250 linear the section of this artificially heated 

 rock still appears as a clear glass, but trichites similar to those present 

 in the unaltered obsidian are again seen; they are, however, much 

 more numerous. A vesicular structure still exists, and the sections of 

 the vesicles are sometimes circular, at others oval. Two or three 

 porphyritic felspar crystals occur in this section, one of them, 

 apparently twinned on the Carlsbad type, has a very irregular outline, 

 somewhat like that of a comb with broken teeth. Felspar crystals 

 with equally irregular contours occur, however, in the unaltered 

 rock. 



In this specimen the devitrification has been confined to the forma- 

 tion of the white crystalline pellets, the rest is glass, containing 

 trichites and, globulites, which of course maybe regarded as evi- 

 dences of incipient devitrification. Still they are also present in the 

 unaltered rock, and between the two sections the differences are 

 barely appreciable, even nnder the microscope. Tigs. 7 and 8 

 (Plate 4) show how close the resemblance is. 



Being anxious to ascertain the result of dry heat upon basic as well 

 as highly silicated glassy rocks, a small fragment of the very vesicular 

 basalt-glass from Mokua Weo Weo, Sandwich Islands, was treated in 

 the same manner as the previously described specimens. This 

 became completely disintegrated in the process of section-cutting. 



