278 
THE TERTIARY VOLCANOES 
BOOK VIII 
that rock, though westward, owing to the thinning away of the undermost 
basalts, the tuff comes to lie directly on the Chalk. Hence, we may legiti- 
mately infer that in this neighbourhood one or more other vents supplied 
the sheets of the lower basalts. 
In the island of Mull a number of detached bosses or patches of 
agglomerate much obscured by invasions of granophyre probably mark the 
sites of volcanic vents. They will be more particularly noticed in Chapter 
xlvii. One of their most interesting features is the large number of 
fragments of felsitic or rhyolitic rocks which they contain. 
In the promontory of Ardnamurchan, where the basalt-plateau has been 
invaded and displaced by later intrusions of crystalline rocks, and has like- 
wise been reduced to such a fragmentary condition by denudation, some 
interesting examples of agglomerate necks have been laid bare. One of the 
largest of these occurs on the north shore at Faskadale. Cut open by the 
sea for more than a quarter of a mile, this neck is seen to be filled with 
a coarse agglomerate, composed mainly of basalt-blocks and debris, but 
crowded also with angular and subangular pieces of different close-grained 
andesitic, felsitic and porphyritie rocks belonging to the acid series to be after- 
wards described . 1 Some of these stones exhibit a very perfect flow-structure, 
and closely resemble certain fine-grained, flinty, intrusive rocks in Mull, to 
which allusion will subsequently be made. The matrix of the agglomerate 
is of the usual dull dirty-green colour, but is so intensely indurated that on a 
fresh fracture it can hardly be distinguished from some of the crystalline 
rocks of the locality. The neck is pierced in all directions with dykes and 
veins of basalt, dolerite, andesite, gabbro, and felsitic rocks. Similar intrusions 
continue and increase in numbers farther west until the cliffs become a laby- 
rinth of dykes and veins running through a mass of rocks which appears to 
consist mainly of dull dolerites and fine gabbros. Though the relations of this 
vent to the plateau-basalts are not quite plain, the agglomerate seemed to 
me to rise out of these rocks. At least the basalts extend from Aehateny 
to Faskadale, but, as they are followed westwards, they are more and more 
invaded by eruptive sheets, and assume the indurated character to which I 
have already referred. 
On the south side of the peninsula of Ardnamurchan, another ag- 
glomerate, noticed by Professor Judd , 2 rises into the bold headland of 
1 One of these felsites when viewed under a high magnifying power is seen to present an 
abundant development of exceedingly minute micropegmatite arranged in patches and streaks 
parallel with the lines of flow-structure in the general cryptocrystalline groundmass. The close 
relationship between the felsites, quartz-porphyries, and granophyres will be afterwards pointed 
out in the description of the acid rocks. It is remarkable that, though these rocks occur 
abundantly in fragments in the volcanic necks and agglomerates of the plateaux, not a single 
instance has been observed of their intercalation as contemporaneous sheets among the basic 
lavas. The analogous case of the interstratification of felsitic tuffs among basic lavas in the 
volcanic series of the Old Red Sandstone of Central Scotland has been described (vol. i. p. 279). It 
is interesting to note that liparitic pumice and dykes have been erupted by some of the basaltic 
craters of Iceland, for example at Askja, Oraffajokull and Snaefellsjokull. (Mr. Thoroddsen, 
Dansk. Geograf. Tidsskrift , vol. xiii. 7th and 8th parts. 
1 Quart. Journ. Geol. Soc. xxx. (1874), p. 261. Professor Judd has subsequently (op. cit. 
xlvi. 1890, pp. 874 et seq.) given a map, section and description of what he believes to be the 
