406 ARRAN. GEOLOGY. OVERLYING ROCKS. 



occasionally present a beautifully radiated fibrous struc- 

 ture j the radii sometimes diverging from a point, and at 

 others from a solid nucleus, which is further in some 

 instances surrounded by a white earthy crust. In the 

 progress of induration the rock at length loses its cha- 

 racter entirely, appearing to pass into a substance of an 

 undefinable nature; of a horny aspect and dark dull 

 green colour. It is possible to convey an idea of this 

 substance by saying that it partakes of the characters both 

 of chalcedony and of pitchstone : it has been sometimes 

 improperly described by the name of globular pitchstone ; 

 a rock from which it is nevertheless far removed, by 

 its extreme toughness, by the want of lustre, and by the 

 form of its fracture. In these hard varieties, the circum- 

 stance formerly described as found in the concretionary 

 siliceous schists of Scalpa and the Shiant isles, also occurs. 

 The intermediate parts of the stone waste by the action 

 of the water, the globular concretions remaining in a 

 distinct form, and producing a botryoidal surface. 



A third vein of the same porphyry occurs still nearer 

 to Clachland point, but it is of smaller dimensions, and 

 seems attended by no peculiarities. 



A remarkable vein of a similar indurated claystone, is 

 found near Machrie on the sea shore, lying between the 

 mouths of the Machrie and the lorsa. It runs nearly 

 at right angles through the sandstone, which is here hori- 

 zontal; being about twenty feet wide, and divided into 

 irregular columns. The point of junction between this 

 vein and the sandstone is not visible. But such junctions 

 may be seen in many places on this side of the island, 

 both at the contact of veins and of overlying masses. 

 One of the most remarkable, occurs near Blackwater bay, 

 eastward of Drumodune. A great continuous mass of 

 the felspar rock, resembling a bed, is here to be seen ; its 

 mineralogical character undergoing, in different places, 

 some slight changes which it is unnecessary to parti- 

 cularize. At one of the extremities, its junction with the 





