28 o 
THE TERTIARY VOLCANOES 
BOOK VIII 
An interesting feature at this locality is the peculiar grouping of some 
of the large dykes in the area around the .agglomerate. They run in the 
direction of the vent, and one or other of them may represent the fissure or 
fissures on which the volcanic orifice was blown open to the surface. 
Another notable element in the geological structure of the ground is the 
vast amount of intrusive material, both in dykes and sheets, which has been 
erupted. The intrusive sheets of Ben Hiant form the most prominent 
eminence in this part of Ardnamurclian. Beserving them for description in 
the following Chapter (p. 3 1 8), I will only remark here that they partly overlie 
the agglomerate, and are therefore, to some extent at least, younger than the 
vent. They belong to that late stage in the history of the basalt-plateaux 
when the molten material, no longer getting ready egress to the surface, 
forced its way among the rocks about the base of the bedded basalts, and 
more especially on the sites of older vents, which were doubtless weak places, 
where it could more easily find relief. 
The large neck now described is only one of a group scattered around 
it in the ground to the north. Two of these may be seen rising through a 
detached area of Jurassic limestones and shales at the northern base of 
Ben Hiant. A third, almost obliterated by the intrusive sheets, may be traced 
at the western end of that mountain above Coiremhuilinn. Two others 
rising through the schists on either side of Beinn na h-Urchrach, have been 
much invaded by the sills of that eminence (Fig. 326). It is doubtless owing 
to the extensive denudation of the basalt-plateau, and the consequent uncover- 
ing of the rocks underneath it, that this series of vents has been laid bare. 1 
By far the largest mass of agglomerate in any of the Tertiary volcanic 
areas of Britain is that which occurs on the north side of the main valley 
of Strath, in Skye. 2 Unfortunately, it has been so seriously invaded 
by the eruptive rocks of the Bed Hills, that its original dimensions and 
its relations to the surrounding rocks, especially to the bedded basalts, are 
much obscured (see Fig. 348). It can be followed continuously from the lower 
end of Loch Kilclirist along the southern slopes of Beinn Dearg Blieag round 
of the Jurassic series and the bottom of the volcanic plateau in this district. On the south and 
west sides of Ben Hiant the Jurassic conglomerates may be seen lying on the edges of the 
crystalline schists only a. little above high-water mark, while on the north side, the schists, 
with their overlying unconformable cake of limestones, rise several hundred feet above sea-level. 
The surface on which the basalts were poured out was probably very uneven, hut there may also 
have been some considerable displacements of these basalts either before or during the injection 
of the dolerite sills of Ben Hiant. 
1 Professor Judd has united these scattered vents into a continuous platform of volcanic 
agglomerates, which he represents as underlying the supposed lavas of Ben Hiant. Since the 
publication of his map and description, I have re-examined the ground without being able to discover 
any trace of this platform. All the visible agglomerates are separate necks, their actual walls 
being sometimes exposed, as in the neck immediately north of the base of Ben Hiant, where the 
limestone in contact is marmorised, though twelve yards oil’ it is an ordinary dull blue rock. 
This extensive mass was not separated from the “ syenite ” of the Red Hills by Maceulloch. 
Ton Oeynhausen and Von Dechen noticed it as a conglomerate with quartz pebbles, but did not 
realise its volcanic nature ( Karslen's Archiv, i. p. 90). In my map of Strath {Quart,. Jour. Geol. 
Soc. xiv. plate i.) I distinguished it from the rock of the Red Hills, but no name for it appears 
in tlie legend of the map, nor is it referred to in the text. Its character as a true volcanic 
agglomerate was recognised by Professor Judd, op. tit. p. 255. See postea , pp. 384 et seq. 
