115 



fore, that the pitchstone mass, like the porphyry, is a transported 

 block, borne upwards from the beds below, and deposited here in 

 shelter of the ridge. Along the southern slopes of the ridge we 

 found many granite blocks strewing the surface. 



The*lower pitchstone vein, c, is fourteen feet thick, and of the 

 same colour and structure as the other. The vein of porphyry occu- 

 pies a ledge about fifty yards lower down : this ledge being on the 

 level of the'! front ^of Dun-Dhu, from which the porphyry extends. 

 The sandstone ^strata are not well seen on the ledges a a a, between 

 the veins, in the line of section ; but come out distinctly enough on 

 their continuations east and west. 



Neither of the pitchstone veins has any connection with those 

 upon* the shore, which are far below the level of the base of our sec- 

 tion. Mr. Headrick, in his work on Arran, says (p. 78) : " Pitch- 

 stone is seen jutting through the soil in various places to the N. and 

 N.W. of Dun-Dhu. In one place near a small burn it rests on red 

 sandstone, and makes the same angle with it." We did not visit 

 this spot ; but it cannot be a continuation of our veins, as these are 

 cut off by the Dun-Dhu porphyry. 



The northern front of Dun-Dhu consists of fine columns of por- 

 phyry, divided at irregular intervals by flat joints. On the N.W. 

 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 por- 

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

 and in a direction from N.E. to S.W., so that their ends, cut uni- 

 formly off, form a wall towards the west, ten to fifteen feet in 

 height. Some of these columns are twenty feet in length, appa- 

 rently without joints : in general, however, 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, occasion- 

 ally ironshot 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 fourteen 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. Across this plateau our path homeward is by the tortuous 

 lane which enters on the Lamlash road immediately below the Free 



