CHAP. XLI1 
THE BASIC SILLS OF ANTRIM 
301 
ammonites and other organic remains have not been destroyed. They are 
preserved in pyrites. 
Of all the examples of Tertiary sills in Britain few are 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 foi 
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 jiorth, it stands upon a talus of blocks as a sheer vertical wall, 250 
Fig. 315.— View of Fair Head, from tlio east, showing the main upper sill and a thinner sheet 
cropping out along the talus slope. 
feet high, and the rude prisms into which it is divided are continuous from 
top to bottom (Fig. 315). So regular is this prismatic structure, and so 
much does it recall the more minute columnar grouping of the bedded 
basalts, that at a little distance we can hardly realize 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 (Fig. 316). It 
may here be remarked that this regular prismatic jointing is one of the 
distinguishing features of the large sills, and serves to mark them oft 
from the bedded basalts, even when these have assumed a columnar 
structure. The prisms are much larger than the basalt-columns, and never 
