As already remarked with regard to the massive, rudely-jointed sheets, many of the most perfectly columnar rocks of the plateaux are not superficial lavas, but intrusive sills, bosses or dykes. Conspicuous examples of such sills are displayed on the coast of Trotternish in Skye, and of the bosses and dykes at the eastern end of Canna. To these further reference will be made in the sequel. It is not always possible to be certain that columnar sheets which appear to be regularly intercalated among the undoubted lavas of the volcanic series may not be really intrusive. In some instances, indeed, we can demonstrate that they are so, when after continuing perfectly parallel with the lavas above and below them, they eventually break across them. One of the most remarkable examples of this feature is supplied by the great sill of the south-west of Stromö, in the Faroe Islands, of which I shall give some account in [Chapter xlii]. (Figs. [312], [328], [329]).
3rd. Slaggy or amygdaloidal lavas without any regular jointed structure, but often with roughly scoriform upper and under layers, and tending to decay into brown earthy debris. Some of the upper surfaces of such sheets among the Tertiary basalt-plateaux must have resembled the so-called "Aa" of the Sandwich Islands. A striking example of the structure may be noticed at Camas Tharbernish, on the north coast of the Island of Canna. There the hummocks on the upper surface of a slaggy basalt measure about 15 feet in breadth, and rise about three feet above the hollows between them, like a succession of waves (see [Fig. 259]). The steam-holes are disposed in a general direction parallel to the strike of the hummocks.
Great variety obtains in the size and shape of the vesicles. Huge cavities a foot or more in diameter may occasionally be found, and from such extremes every gradation may be traced down to minute pore-like vacuoles that can hardly be made out even with a strong lens. In regard to the deformation of the vesicles, it is a familiar general rule that they have been drawn out in the direction of the flow of the original lava. Occasionally they have become straight, narrow, sometimes bifurcating pipes, several inches long, and only an eighth of an inch or so in diameter.[222] A number of such pipes, parallel to each other, resembles a row of worm-burrows (see [Fig. 2]).
[222] Some examples have been deposited by me in the Museum of Practical Geology, Jermyn Street, in the case illustrating rock-structures. The elongation of the vesicles into annelide-like tubes may also be observed among the stones in the volcanic agglomerates.
It may often be noticed that, even where the basalt is most perfectly prismatic, it presents a cellular and even slaggy structure at the bottom. The rock that forms the Giant's Causeway, for instance, is distinctly vesicular, the vesicles being drawn out in a general east and west direction. The beautiful columnar bed of Staffa is likewise slaggy and amygdaloidal for a foot or so upwards from its base, and portions of this lower layer have here and there been caught up and involved in the more compact material above it. Even the bottom of the confusedly prismatic bed above the columnar one on that island also presents a cellular texture. A similar rock at Ardtun, in Mull, passes upward into a rugged slag and confused mass of basalt blocks, over which the leaf-beds lie.
Amygdaloidal structure is more or less developed throughout the whole series of basalts. But it is especially marked in certain abundant sheets, which, for the sake of distinction, are called amygdaloids. These beds, which form a considerable proportion of the materials of every one of the plateaux, are distinguished by the abundance and large size of their vesicles. In some places, the cavities occupy at least as much of the rock as the solid matrix in which they lie. They have generally been filled up with some infiltrated mineral—calcite, chalcedony, zeolites, etc. The amygdales of the west of Skye and of Antrim have long been noted for their zeolites. As a consequence of their cellular texture and the action of infiltrating water upon them, these amygdaloidal sheets are always more or less decomposed. Their dull, lumpy, amorphous aspect contrasts well with the sharply-defined columnar sheets above and below them, and as they crumble down they are apt to be covered over with vegetation. Hence, on a sea-cliff or escarpment, the green declivities between the prominent columnar basalts usually mark the place of such less durable bands.
Exceedingly slag-like lavas are to be seen among the amygdaloids, immediately preceded and followed by beds of compact black basalt with few or no vesicles. From the manner in which such rocks yield to the weather, they often assume a singularly deceptive resemblance to agglomerates. One of the best examples of this resemblance which have come under my notice is that of the rock on which stands Dunluce Castle, on the north coast of Antrim. Huge rounded blocks of a harder consistency than the rest of the rock project from the surface of the cliffs, like the bombs of a true volcanic agglomerate, while the matrix in which they are wrapped has decayed from around them. But an examination of this matrix will soon convince the observer that it is strongly amygdaloidal, and that the apparent "bombs" are only harder and less cellular portions of it. The contrast between the weathering of the two parts of the rocks seems to have arisen from an original variety in the relative abundance of steam-cavities. The origin of such nodular or pillow-like blocks has been already referred to at pp. [26] and [193]. Another singular instance occurs at the foot of the outlier of Fionn Chro ([Fig. 360]), in the island of Rum. A conspicuous band underlying the basalts there might readily be taken for a basalt-conglomerate. But in this case, also, the apparent matrix is found to be amygdaloidal, and the rounded blocks are really amygdales, sometimes a foot in length, filled or lined with quartz, chalcedony, &c.
A somewhat different structure, in which, however, the appearance of volcanic breccia or agglomerate due to explosion from a vent is simulated, may be alluded to here. The best instance which I have observed of it occurs at the south end of Loch-na-Mna, in the island of Eigg, within a basalt which is remarkable for a streaky flow-structure. On the weathered faces the streaky layers may be observed to have been broken up, and their disconnected fragments have been involved in ordinary basalt wherein this flow-structure is not developed, while large blocks and irregular masses are wrapped round in a more decomposing matrix. There can be no doubt that in such cases we see the effects of the disruption of chilled crusts, and the entanglement of the broken pieces in the still fluid lava.
It is a common belief that the filling in of the steam-cavities has taken place long subsequent to the volcanic period, by the slow percolation of meteoric water through the rock. I believe, however, that at least in some cases, if not in all, the conversion of the vesicular lavas into amygdaloids was effected during the volcanic period. Thus it can be shown that the basalts which have been disrupted by the gabbros and granophyres were already amygdaloids before these basic intrusions disturbed them, for the kernels of calcite, zeolite, etc., have shared in the general metamorphism induced in the enclosing rock. Again, the blocks of amygdaloid contained in the agglomerates of the volcanic series are in every respect like the amygdaloidal lavas of the plateaux. It would thus seem that the infilling of the cavities with mineral secretions was not merely a long secular process of infiltration from the cool atmosphere, but was more rapidly completed by the operation of warmer water, either supplied from volcanic sources or heated by the still high temperature of the cellular lavas into which it descended from the surface.[223]
[223] Professor J. D. Dana, originally an advocate of infiltration from above, subsequently supported the view that the kernels of amygdaloids were filled in by the action of moisture within the rocks during the time of cooling.—Amer. Journ. Sci. ser. 3, vol. xx. (1880), p. 331. Messrs. Harker and Marr have demonstrated that the Lower Silurian vesicular lavas of the Lake district had already become amygdaloids before the uprise of the Shap granite.—Quart. Journ. Geol. Soc. vol. xlix. (1893).