112 



elongation, passing into them by insensible degrees. The vein next 

 the sandstone varies much in hardness, but it cannot be said that 

 this induration bears any relation to its proximity to the sandstone. 

 The concretionary structure is seen both in the hard and soft 

 varieties, it is radiated fibrous, the radii sometimes diverging from a 

 point, and sometimes 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 character entirelv, appearing 

 to pass into a substance of an indefinable nature, of a horny aspect 

 and dark dull green colour partaking of the character both of cal- 

 cedony and pitchstone. It has been described as globular pitch- 

 stone ; but it is far removed from this rock by its extreme tough- 

 ness, want of lustre, and by the form of its fracture." Mineralo- 

 gists have long regarded this curious rock with much interest, and 

 various opinions have been held respecting its true relations, some 

 considering it as allied to claystone, and others to pitchstone. A 

 comparison of the appearances observed here with those seen at Tor- 

 more, and in Moneymore glen, has led us to conclude, that " the 

 substance of an indefinable nature" is hornstone ; and a transitional 

 state of these earthy matters, between claystone and pitchstone ; 

 the globules being due to the formation, within the fluid mass, of 

 crystalline centres of radiation, in the manner described in Mr. Gre- 

 gory Watt's Experiments (Art. 31). The radiations are of pure 

 quartz, while the base is of the same substance, but mixed with 

 colouring matter, iron or manganese, and other trifling impurities. 

 We shall again notice similar varieties in other places. 



76. Near the pitchstone vein the high ridge of Dun-fion reaches 

 the coast ; and the trap which forms its summit appears in section 

 in the cliffs, overlying the sandstone. It presents a fa$ade of imper- 

 fect columns, and contrasts strongly with the stratified sandstone. 

 Both sink rapidly southward in the direction of the dip ; the sand- 

 stone is depressed below the sea-level, and the trap then occupies 

 the coast, forming the low point at the entrance of Lamlash bay, 

 and the small island in the channel. Near the point where the 

 sandstone disappears, some remarkable effects of trap dikes are exhi- 

 bited. On the west side of the point the sandstone suddenly 

 emerges again, and extends to Lamlash. The whole bay, indeed, 

 has been excavated in this red rock, which forms the base of the 

 cone of Holy Isle, as well as the coast on the mainland. But we 

 turn here meanwhile, and direct our steps to Brodick, by the sum- 

 mit of the ridge, which divides the two bays. Its southern side 

 forms a long grassy slope of gentle inclination towards Lamlash 



