COMMONLY CALLED GREENSTONES. 81 intruded partially along the planes of bedding; at other times crossing them transversely and vertically. In the former case, it often assumes a bedded aspect, and a columnar structure, in which the axes of the columns are at right angles to the planes of cooling ; an example of which may be observed on the north ern flank of Cader Idris mountain in Wales. 1 Diorite is generally extremely hard and tough, and is consequently well suited for road-material and paving. The Penmaenmawr stone of North Wales, which may be called a felspathic greenstone, 2 is shipped to all parts of the British Isles for paving. The rock, which is finely crystalline granular, com posed of felspar and hornblende, is intersected by a great number of parallel joints, along which it splits into small blocks of a size suitable for street paving. Its hardness and durability are extreme. Bardon Hill, in Leicestershire, also affords a stone of similar qualities, but of more variable composition, 3 which is distributed for road metal over a large part of the central counties of England. It is now becoming thoroughly understood that it 1 These modes of occurrence are well illustrated by the maps and sections of the Govt. Geological Survey of Great Britain. 2 Descrip. Catal. Eocks M. P. G., p. 202. 3 This rock varies from a felstone porphyry to a diorite or greenstone, and from this to syenite, composed of hornblende, fel spar, and quartz. Hull, 1 Geology of the Leicestershire Coal-field.’ Mem. Geol. Survey, p. 13, i860.