300 
THE TERTIARY VOLCANOES 
BOOK VIII 
which they exhibit the phenomena of “ segregation- veins ” — -so charac- 
teristic of the thicker and more coarsely crystalline sills. These veins or 
seams here differ from the rest of the rock mainly in the much larger size 
and more definitely crystalline form of their component minerals. Though 
sharply defined, when looked at from a little distance, they are found on 
closer inspection to shade into the surrounding rock by a complete inter- 
lacing of crystals. On the shore, they can be seen to lie, on the whole, 
parallel with the bedding of the sheets in which they occur, but without 
rigidly following it, since they undulate and even ramify. A good section 
across their dip has been exposed in a quarry near the end of the pro- 
montory, and shows that they are considerably less regular than the plan 
of their outcrop on the shore would have led us to anticipate. The accom- 
panying drawing (Fig. 314) represents the veins laid bare on a face of rock 
Fig. 314. — View of “ Segregation- Veins ” in a dolerite sill, Portrusli, Antrim. 
nine feet in length by five feet in height. It will be seen that while there is 
a general tendency to conform to the dip-slope, which is here from right to 
left, the seams or layers unite into a large rudely-bedded mass, which sends 
out processes at different angles. The peculiar aggregation of minerals 
which distinguishes such veins is perhaps best seen at Fair Head, and I 
reserve for the description of that locality what I have to say on the sub- 
ject, only remarking with regard to the Portrusli rock that the felspar 
shows a disposition to collect in the centre of the veins with the augite and 
the other dark minerals at the outer margins. 
The contact-metamorphism at this locality is of more historical interest 
in connection with the progress of geological theory than of scientific » 
importance. It consists mainly in an intense induration of the argillaceous 
strata. These pass here from their usual 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. Tet the 
