82 GEOLOGY OF ARRAN. 



bottle-green colour, homogeneous, and very seldom exhibits 

 any porphyritic structure. Between the two first outbreaks, 

 on the bank of a stream, there is a bed or dike of a very 

 hard and tough hornstone, or brownish gray quartzite, with 

 imbedded bits of quartz, a slight modification merely of the 

 pitchstone base; there is a greenstone dike beside it. The 

 relations of these igneous products are very curious. 



42. The northern front of Dun-Dhu consists of fine 

 columns of porphyry, divided at irregular intervals by flat 

 joints. On the north-west side the columns are very perfect, 

 and have a singular diverging fan-shaped arrangement. We 

 ascend the hill easily from the south side, and find the 

 summit composed of huge prismatic masses of porphyry, 

 lying closely side by side almost in a horizontal position, and 

 in a direction from north-east to south-west, so that their 

 ends, cut uniformly off, form a wall towards the west, ten to 

 fifteen feet in height. Some of these columns are twenty 

 feet in length, apparently without joints : in general, how- 

 ever, there is an indication of a flat jointing, at distances of 

 four to seven feet. Most of the prisms are four and five- 

 sided. The rock has a gray felspar base, occasionally iron- 

 shot throughout, or merely streaked with iron; and the 

 imbedded crystals are of glassy felspar. Bits of quartz are 

 also disseminated through the base apparently minute 

 crystals with their angles rounded off, as if by attrition. 



No granite blocks were noticed on this detached summit; 

 but many of considerable size, as much as from six to four- 

 teen tons, were observed close to the cliffs on the west side. 

 They are found in great numbers, and of large size, over all 

 parts of the open plateau between the base of this ridge and 

 the shores of Brodick Bay. They have been mostly blasted 

 hereabouts, gathered off the fields, and reared into fences. 

 Many of them are rounded, polished, and marked with very 

 perfect glacial strise traced upon them ere yet detached 

 from their native beds in the granite nucleus, or else in their 

 transit upon the ice to their present resting-place. Across 



