CHAP. XXXVI 
STRUCTURE OF THE BASALT-PLATEAUX 
187 
Fig. 259. — Section of scoriaceons and prismatic Basalt, Camas 
Tharbernish, north shore of Ganna Island. 
at Staffa. Excellent illustrations of both these types may be seen at many 
points along the sea-cliffs of the Inner Hebrides ; the western coast of Skye, 
the south-west side of Mull, and the cliffs of the island of Canna may be 
specially cited. 
Though generally rather compact, becoming indeed dense, almost 
vitreous rocks in some 
sheets, the columnar 
basalts are often more 
or less cellular through- 
out, and highly slaggy 
along their upper and 
under surfaces. In some 
cases, as in that of a 
prismatic sheet which overlies the rough scoriaceous lava of Camas Tharber- 
nish, in the island of Canna, the rows of vesicles are disposed in lines parallel 
to the under surface of the sheet (Fig. 259.) 
As already remarked with regard to the massive, rudely-jointed sheets, 
many of the most perfectly columnar rocks of the plateaux are not super- 
ficial 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 Stromo, 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 
mto 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 
m 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 
