438 



Mr. F. Kutley. 



[May 27, 



the latter locality, where the lavas are chiefly of a basic character, may 

 be simply altered fragments of vesicular basalt-glass. 



The following specimens were prepared some years since by Mr. 

 Gr. F. Rodwell, late Science Master in Marlborough College. Fig. 3, 

 Plate 5, represents part of a section made from a black vesicular glass 

 which resulted from the fusion of some of the basalt from the Giants' 

 Causeway, in an ordinary Stourbridge-clay crucible, over a gas furnace. 

 The mass was then rapidly cooled. The glass appears perfectly clear, 

 and shows merely vesicles and irregular cracks. Another sample of the 

 same rock was also fused in the same manner, but was allowed to cool 

 slowly. A section cut from this (fig. 4, Plate 5) is translucent only 

 on certain edges, where a prismatic structure is visible, the marginal 

 portions of the prisms showing a radiating crystalline or fibrous 

 character as indicated in the drawing. The rest of the section is 

 opaque, as in the altered Kilauea lava. It may be added that a some- 

 what similar prismatic structure is occasionally to be seen in opaque 

 specimens from the Sandwich Island lavas. Among Mr. Rodwell's 

 numerous experiments he placed a fragment of cold basalt from the 

 Griants' Causeway upon the surface of a molten mass of the same rock 

 and allowed it to sink to the bottom of the crucible. Figs. 5 and 6, 

 Plate 5, represent parts of a section taken through the enveloped 

 fragment and the enveloping glass. The latter is quite clear, as 

 shown on the right of fig. 6. On the left of this figure will be seen a 

 belt of opaque matter, which intervenes between the fragment of 

 basalt, seen on the left of fig. 5, and the clear glass on the right of 

 fig. 6 ; while on the right of fig. 5 we see more of this opaque belt 

 where it comes in contact with the enveloped fragment of basalt. The 

 boundaries of this opaque belt are sharply defined, especially where it 

 adjoins the clear glass. Within this dark belt are numerous radiating 

 groups of colourless, transparent, acicular crystals, the terminations 

 of some of them being shown in the dark half of fig. 5, Plate 5. 



Whether the formation of this opaque zone is due to the chilling 

 influence of the cold fragment on the hot magma into which it sank, 

 is a question which may have its bearings upon the formation of 

 tachylyte at the sides of intrusive veins and dykes of basalt where 

 the molten mass has come in contact with the walls of a fissure. It 

 may also be that the fragment of basalt parted with its heat less 

 rapidly than the surrounding magma, and that the latter consequently 

 cooled more slowly where it came in contact with the fragment. This 

 seems at least a plausible explanation, and harmonises with the results 

 of Mr. Rodwell's earlier experiments, shown in figs. 3 and 4, Plate 5. 



Additional investigations will be made upon the rocks treated of in 

 this paper, since the results hitherto obtained have been effected 

 under conditions which may not be regarded as the most favourable 

 for producing changes similar to those which take place in nature, 



