RUM. GEOLOGY. 



used for paving ; while nearer the imaginary centre of the 

 mass, they put on the semblance of frusta of pyramids, 

 rather than that of the joints of basaltic columns, to which 

 it is obvious that they have a near affinity. No drawing 

 can give an adequate idea of this strange appearance, but 

 a conception of it may be formed by supposing that a 

 fluid substance had congealed at the surface, while the 

 internal parts were still in a fluid state ; and that the crust 

 thus formed had been broken into fragments by the 

 expansion of the central parts. Similar appearances can 

 in fact be produced by the cooling of certain metals and 

 other bodies from a state of fusion. Whether from the 

 greater novelty of this appearance, or its more extraor- 

 dinary aspect, it produces a more striking effect than any 

 other basaltic display which I have witnessed. The uni- 

 formity of the surface, the depth of the fissures, the mag- 

 nitude of the polygons, and the great scale of the whole, 

 together with the decision of the parts, give it an aspect 

 resembling much more a gigantic effort of art than the 

 columnar pavements either of Staffa or of the Giant's 

 Causeway. The resemblance which it bears to the argillo- 

 calcareous septaria will, on a general view, suggest itself 

 to an observer ; but it is necessary to remark that this set 

 of fissures is arranged in a reverse order, the widest inter- 

 vals lying on the outside. It is unnecessary to inquire 

 how far it is connected with the theories respecting the 

 formation of trap, as the analogies will be obvious to 

 every geologist. 



This rock is remarkably sonorous when struck, rendering 

 a sound precisely like that of iron. It is of a dark blue 

 colour and minute granular texture, containing augit dis- 

 persed through it in a particular form, which I have 

 already mentioned as occurring in the Shiant isles, as 

 well as in the granular augit rock of this island, and 

 which can in no way be so well described as by comparing 

 it to the skeletons of large crystals. In a fortunate frac- 



