EXCURSION II. 77 



stone varies much in hardness, but it cannot be said that this 

 induration bears any relation to its proximity to the sand- 

 stone. 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 entirely, 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 calcedony 

 and pitchstone. It has been described as globular pitch- 

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

 toughness, want of lustre, and by the form of its fracture." 

 Mineralogists 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 Tormore, and in Moneymore 

 glen, has led us to conclude, that " the substance of an in- 

 definable nature " is homstone ; and a transitional state of 

 these earthy matters between claystone and pitchstone ; the 

 globules being due to the formation, within the mass when in 

 a fluid state, of those crystalline centres of radiation, which 

 have been observed to exist in melted rock, and which appear 

 to determine the prismatic and jointed structure when the 

 fused mass cools slowly. 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. (See Excursion XI., Art. 72.) 



40. 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 clifis, overlying the sandstone. It 

 presents a facade of imperfect columns, and contrasts strongly 

 with the stratified sandstone. Both sink rapidly southward 

 in the direction of the dip; the sandstone is depressed below 



