242 Proceedings of the Royal Physical Society. 



sharp. The vitreous rock stands up prominently as a wall, 

 on the outer surface of which a curious series of polygonal 

 reticulations may be observed. These, as shown on PL V., 

 fig. 2, consist of prominent ribs, each about ^ a couple of 

 inches broad, enclosing shallow, cup-shaped hollows. This 

 external sculpturing arises from the internal prismatic struc- 

 ture of the vitreous band. The prisms are, of course, ap- 

 proximately horizontal, being directed from the nearly ver- 

 tical walls of the band. Each consists of a central core of 

 vitreous rock, with an external sheath of dull and, to the 

 eye, apparently more crystalline and devitrified rock. It 

 is the union of the sheaths of contiguous prisms which has 

 formed these prominent ribs. Each rib is thus composed 

 of the outer shell of two prisms, the dividing line between 

 which is marked by a suture along the centre of the rib. 

 Moreover, each rib is cut into small segments by a system 

 of close joints, which are placed generally at a right angle 

 to the course of the rib. (Plate V., figs. 2, 3, and 4.) 



On breaking open the vitreous cores of the prisms, we 

 perceive that the rock, deep iron-black in colour, has a 

 peculiar vitreous glaze quite unlike that of an. ordinary 

 basalt ; that here and there the black glass of the base is 

 segregated into kernels or irregular patches, in which can 

 generally be detected one or more amygdules of chalcedony, 

 but some of which have a black empty internal cavity, and 

 that throughout the rock devitrification has proceeded so far 

 as to allow of the individualisation of the triclinic felspar 

 (of which distinct facettes can be seen with a lens), and 

 evidently also of other minerals. Occasionally rounded green 

 grains may be observed, which recall the olivines of common 

 basalt. The rock is remarkably close-grained, breaking with 

 a splintery, sub-conchoidal fracture. It is tolerably homo- 

 geneous in texture, the most prominent feature being the 

 patches of black glass of all sizes up to that of a small bean, 

 and the occasional amygdules of dull white chalcedony. 



The hard prominent ribs which divide the vitreous cores 

 along the margin of the central band of the dyke, present 

 to the naked eye a texture which recalls that of many some- 

 what decayed felsites. The rock, instead of the vitreous 



