Fig. 315.—View of Fair Head, from the east, showing the main upper sill and a thinner sheet cropping out along the talus slope.
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 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 vertical wall, 250 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 off from the bedded basalts, even when these have assumed a columnar structure. The prisms are much larger than the basalt-columns, and never display the irregular starch-like arrangement so common among the plateau-basalts.
Fig. 316.—View of Fair Head from the shore. (From a Photograph by Mr. R. Welch.)
The rock composing this magnificent sheet is a coarsely crystalline, ophitic, olivine-dolerite.[314] 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, Carboniferous 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 supervenes. A similar closeness of grain is observable at the surfaces of contact where the sheet splits up on its western border.
[314] Professor Judd has described what he calls a "glomero-porphyritic structure" in this rock (Quart. Journ. Geol. Soc. xlii. (1886), p. 71).
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 of the rock, the felspar and augite showing the ophitic intergrowth of the main rock, but on a far larger scale. Some of the pieces of augite measure two inches or 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 described 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 other, have forced their way between and partly across the bedding planes of the Carboniferous shales ([Fig. 317]). In this way the huge, unbroken mass, 250 feet thick, subdivides itself and disappears in a few hundred yards, though it continues a little further inland, and approaches the shore again half a mile to the south-west. Further evidence of the intrusive nature of this rock may be observed along the base of the precipice, where at least one sheet 70 feet thick diverges from the main mass and runs eastwards between the Carboniferous shales ([Fig. 315]). At the contact with the eruptive rock the shales are everywhere much indurated.
Fig. 317.—Section at Farragandoo Cliff, west end of Fair Head, showing the rapid splitting up and dying out of an Intrusive Sheet.
a, Carboniferous sandstone; b, Carboniferous shale; c, intrusive sheet.