114 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



condition of fissile, laminar, dull, dark shales into an exceedingly compact, black, flinty 

 substance, which in its fracture, colour and hardness reminds one of Lydian stone. Yet 

 the ammonites and other organic remains have not been destroyed. They are preserved 

 in pyrites. 



Of all the examples of intrusive sheets of Tertiary age in Britain there is none more 

 imposing than that of the noble range of precipices which form the promontory of 

 Fair Head. Leaving out of account the minor masses of eruptive rock which occur 

 underneath it, we find the main sheet to extend along the coast for nearly four miles, 

 to rise to a height of 636 feet above the sea, and to attain a maximum thickness of 250 

 feet. This enormous bed dies out rapidly both to the east and west, and seems also to 

 thin away inland. Seen from the north, it stands upon a talus of blocks as a sheer 



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-^ -"l ^<r — - — ^ x ^ ^ \ ,v 



, V X^^^=^^^ 



Fig. 33. — View of Fair Head from the East, showing the main upper intrusive sheet and a thinner sheet cropping out along 



the talus slope. 



vertical wall, 250 feet high, and the rude prisms into which it is divided are continuous 

 from top to bottom (fig. 33). So regular is this prismatic structure, and so much does 

 it recall the more perfect columnar grouping of the basalts, that at a little distance we 

 can hardly realise the true scale of the structure. It is only when we stand at the base 

 of the cliff or scramble down its one accessible gully, the " Grey Man's Path," that we 

 appreciate how long and thick each of the prisms actually is. 



The rock composing this magnificent sheet is a coarsely crystalline, ophitic, olivine- 

 dolerite.* The same diminution of the component crystals, which is so marked along the 



* Professor Judd has described what he calls a " glomero-porphyritic structure" in this rock (Quart. Jour. Geol. 

 .W., xlii. (1886) p. 71). 



