CHAP. XLII 
THE BASIC SILLS OF ANTRIM 
303 
display the irregular starch -like arrangement so common among the 
plateau-basalts. 
The rock composing this magnificent sheet is a coarsely crystalline, 
ophitic, olivine-dolerite. 1 The same diminution of the component crystals, 
which is so marked along the margins of the eruptive masses at Portrush, is 
strikingly exhibited at the borders of the Fair Head sill. For about 18 or 
20 inches upward from the bottom, where the bed rests on the black, Car- 
boniferous shales, the dolerite is dark and finely crystalline, weathering 
spheroidally in the usual manner. But immediately above that bottom 
layer of closer grain, the normal coarsely crystalline texture rapidly super- 
venes. A -similar closeness of grain is observable at the surfaces of contact 
where the sheet splits up on its western border. 
Nowhere, so far as I know, can the phenomena of “ segregation- veins ” 
be so instructively studied as along the abundant exposures of this great 
sheet. The veins are most conspicuous where the rock occurs in thickest 
mass. They vary up to three or four feet in thickness, and, as at Portrush 
and elsewhere, lie on the whole parallel to the upper and under surfaces of 
the sheet. An erroneous impression may be conveyed by the term “ veins ” 
applied to them. They are quite as much layers, parallel on the whole with 
the bedding of the sheet, yet not adhering rigidly to one plane, but passing 
across here and there from one horizon to another. That they are not due 
to any long subsequent protrusion of younger material through the main 
sheet is made manifest by the thorough interlocking of their component 
crystals with those of the body of the rock in which they lie. The material 
that fills these veins has obviously been introduced into them while there 
was still some freedom of movement among the crystals of the surrounding 
rock, which must thus have been still not quite consolidated and therefore 
intensely hot. Both crystallized slowly, and in so doing their component 
minerals dovetailed with each other. The constituents of the veins consist 
of an exceedingly coarse aggregate of crystals, or rather of crystalline lumps 
of the same minerals that constitute the general mass oi the rock, the 
felspar and augite showing the ophitic intergrowth ol the main rock, but 
on a far larger scale. Some of the pieces of augite measure two inches 01 
more in diameter. The conditions under which these veins were produced 
must have differed in some essential respects from those that prevailed 
during the formation of the fine-grained, highly siliceous veins already 
deseribed as occurring in some dykes and sills. 
This great Fair Head sill lies upon Carboniferous strata, but that it is 
to be classed with the Tertiary volcanic series is, I think, demonstrated by 
its relations to the Chalk at its eastern end. It has. there broken through 
that rock, and converted it for a short distance into, a white, granular 
marble. But it is at the western side that the most interesting sections 
occur to show the truly intrusive nature of the mass. The rock there splits 
up into about a dozen sheets, which, keeping generally parallel with each 
1 Professor Judd lias described what lie calls a “ glomero-porpliyritic structure” in this rock 
{Quart. Journ. Geol. Soc. xlii. (1886), p. 71). 
