2.34 
THE TERTIARY VOLCANOES 
BOOK VIII 
and allowed to retain its channel long enough to he able to erode it to a 
depth of nearly 5 0 feet. Erosion had reached down through the underlying 
tuffs to the slaggy basalt below, but before it had made any progress in 
that sheet its operations were brought to an end at this locality by the floods 
that swept in the coarse shingle, and by the subsequent stream of basalt of 
which a mere outlying fragment now forms the upper third of the stack 
(«, Kg- 274). 
That the ravine or gully of Dim Beag probably lay within the reach of 
the floods of the main river, may be inferred from the number and size of 
the far- transported rocks in its conglomerate. It was filled up gradually, 
but the conditions of deposition remained little changed during the process, 
except that the largest blocks of rock were swept into the chasm in the 
earlier part of its history, while much smaller and more water-worn shingle 
were introduced towards the close. 
Denudation, which has performed such marvels in the topography of 
the West of Scotland since older Tertiary time, has here obliterated every 
trace of this ancient gully, save the little fragment of one of the walls which 
survives in the stack of Dun Beag. When in the course of centuries this 
picturesque obelisk shall have yielded to the action of the elements, the last 
leaflet of one of the most interesting chapters in the geological history of the 
Inner Hebrides will have been destroyed. 
The question naturally arises — What was the subsequent history of the 
liver which has left so many records of its floods entombed among the 
basalts of Ganna and Sanday ? In particular, can any connection be traced 
or plausibly conjectured between it and the river-bed preserved under the 
Seuir of Eigg ? To this question I shall return after the evidence for the 
existence and date of the latter stream has been laid before the reader. 
In the. chain of the Inner Hebrides, broken as it is in outline and varied 
in its types of scenery, there is no object more striking than the island of 
Eigg. Though only about live miles long and from a mile and a half to 
three miles and a half broad, and nowhere reaching a height of so much as 
1300 feet, this little island, from the singularity of one feature of its 
surface, forms a conspicuous and familiar landmark. Viewed in the 
simplest way, Eigg may be regarded as consisting of an isolated part of 
the basaltic plateau which, instead of forming a rolling tableland or a 
chain of hills with terraced sides, as in Antrim, Mull and Skye, has been so 
tilted that, while it caps a lofty cliff about 1000 feet above the waves at the 
north end, it slopes gently along the length of the island to the south end. 
In its southern half, however, the ground rises, owing to the preservation 
of an upper mass of lavas, which denudation lias removed from the northern 
half. On this thicker part of the plateau stands the distinguishing 
feature of the island, the strange fantastic ridge of the Scuir, which, seen 
from the north or south, looks like a long steep hill-crest, ending in a sharp 
precipice on the east. Viewed from the east, this precipice is seen to be the end 
of a huge mountain-wall, which rises vertically above the basalt-plateau to 
a height of more than 350 feet. The accompanying map (Fig. 2*76) shows 
