CHAPTER XLII 
THE BASIC SILLS OF THE BASALT-PLATEAUX 
We have now followed the distribution of the basalt-plateaux, the arrange- 
ment of their component materials which were erupted at the surface, and 
the character of the dyke-fissures and vents from which these materials were 
ejected. But there remains to be considered an extensive series of rocks 
which display some of the underground phenomena of the Tertiary volcanoes. 
The injection of many basaltic sheets had been clearly enforced by Mac- 
culloch. In 1871 I pointed out that at different horizons in the plateau- 
basalts, but especially at their base and among the stratified rocks underneath 
them, sheets of basalt and dolerite occur which, though lying parallel with 
the stratification of the volcanic series, are not truly bedded, but intrusive, 
and therefore younger than the rocks between which they lie. 1 The non- 
recognition of their true nature had led to their being regarded as proofs of 
volcanic intercalations in the Jurassic series of Scotland. There is, however, 
no trace of the true interstratification of a volcanic band in that series, 
every apparent example being due to the way in which intrusive sheets 
simulate the characters of contemporaneous flows. 
If such sheets had been met with only at one or two localities, we might 
regard them as due to some mere local accident of structure in the overlying 
crust through which the erupted material had to make its way. But when 
we find them everywhere from the cliffs of Antrim to the far headlands of 
Skye and the Shiant Isles, and see them reappear among the Faroe Islands, it 
is obvious that, like those of Paheozoic time, they must be due to some 
general cause, and that they contain the record of a special period or phase 
in the building up of the Tertiary volcanic tablelands. I will first describe 
some typical examples of them from different districts, and then discuss 
their probable relations with the other portions of the plateaux. 
i. ANTRIM 
First to be examined, and now most familiar to geologists, are the 
remarkable sheets that underlie the plateau of Antrim, and project at various 
parts of the picturesque line of coast between Portrush and Fair Head. 
■ 1 Quart. Jour. Geol. Soc. xxvii. (1871), p. 296. 
