1 88 
THE TERTIARY VOLCANOES 
BOOK VIII 
inches long, and only an eighth of an inch or so in diameter. 1 A number of 
such pipes, parallel to each other, resembles a row of worm-burrows (see 
Fig. 2). 
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 agglomer- 
ates. 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 
1 Some examples have been deposited by me in the Museum of Practical Geology, Jermyu 
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. 
