In the destruction of the precipice some of the vents have been so much cut away that only a small part of the wall is left, with a portion of the agglomerate adhering to it. The third neck, for instance, affords the section represented in [Fig. 313], where the horizontal sheets of basalt (a) have still a kind of thick pellicle of the volcanic detritus (b) adhering to what must have been part of the side of the orifice of eruption. The waves have cut out a cave at the base, so that we can, by boat, get behind the agglomerate and see the margin of the volcanic funnel in the roof overhead.
The fragment of geological history so picturesquely laid bare on the Stromö cliffs presents a significant illustration of what seems to have been a frequent, if not the normal type of volcanic vent in the Tertiary basalt-plateaux. By the fortunate accident that denudation has not proceeded too far, we are able to observe the original tops of at least two of the vents, and to see how such volcanic orifices, which were doubtless abundant all over these plateaux, came to be entombed under the ever-increasing pile of accumulating basalt.
There is still one feature of interest in these cliff-sections which deserves notice here. Every geologist who has studied the composition of the basalt-plateaux has remarked the comparatively insignificant part played by tuffs in these volcanic accumulations. Hundreds of feet of successive basalt-sheets may often be examined without the discovery of any intercalation of fragmental materials, and even where such intercalations do occur they are for the most part quite thin and extremely local. I found it impossible to scale the precipice for the purpose of ascertaining whether around the Stromö vents, and connected with them, there might not be some beds of tuff interstratified between the basalts. If such beds exist, they can only be of trifling thickness and extent. Here, then, are examples of once active vents, the funnels of which are still choked up with coarse fragmentary ejections, yet from which little or no discharge of ashes and stones took place over the surrounding ground. They seem to have been left as crater-like hollows on the bare surface of the lava-fields.
CHAPTER XLII
THE BASIC SILLS OF THE BASALT-PLATEAUX
We have now followed the distribution of the basalt-plateaux, the arrangement 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 Macculloch. 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.[309] 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.
[309] Quart. Jour. Geol. Soc. xxvii. (1871), p. 296.
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 Palæozoic 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. From the shore at Portrush, as I have already remarked, came the evidence that was supposed to prove basalt to be a rock of aqueous origin, inasmuch as shells were obtained there from what was believed to be basalt. The long controversy to which this supposed discovery gave rise is one of the most curious in the history of geology.[310] It continued even after the illustrious Playfair had shown that the pretended basalt was in reality highly indurated shale, and hence that, instead of furnishing proof of the aqueous formation of basalt, the Portrush sections only contributed another strong confirmation of the Huttonian theory, which claimed basalt to be a rock of igneous origin.