232 
THE TERTIARY VOLCANOES 
BOOK VIII 
This extraordinary monument of geological history forms an outlying obelisk 
which rises from the platform of the shore to a height of about 7 0 feet. 
Seen from the south-west, it appears to consist entirely of bedded basalt 
resting on some stratified tuff and shale which intervene between these lavas 
and that of the broad platform of basalt on which the obelisk stands. On 
that side it presents no essential difference from the structure of the Dim 
Mor to the west, save that the lower conglomerate of that outlier is here 
represented by fine sediment, and the upper conglomerate is wanting. The 
general aspect of this south-western front of the stack is shown in Fig. 272. 
Tf, however, we approach the rock from the coast-gully to the north, we 
Fig. 274. — Section of eastern front of Dun Beag. 
a f Very shaggy amygdaloidal basalt ; b, shales and tuff ; c, slaggy and jointed basalts ; d, conglomerate ; 
e, prismatic basalt. 
The dotted lines indicate the supposed form of the ravine. 
form a very different impression of its structure. It then appears to con- 
sist chiefly of conglomerate with a capping of basalt on the top (Fig. 
273). Not until a close scrutiny is made of the eastern and western 
faces of the column do the true structure and history of this singular 
piece of topography become apparent. 
On the eastern front, the section represented in Fig. 274 is exposed. 
At the bottom, forming the pediment of the column, lies a sheet of slaggy 
and vesicular or amygdaloidal basalt (a), which shelves gently in a south- 
westerly direction into the sea. The lowest band (b) in the structure of the 
stack is a thin group of lilac, brown, and green shale and volcanic mudstone 
or tuff, which encloses pieces of coniferous wood, and becomes markedly 
carbonaceous in its uppermost layers. Above these strata on the south 
