CHAP. XLI 
VENTS OF THE BASALT-PLATEAU X 
281 
to the western roots of Beiim Dearg Mhor — a distance of more than two 
miles in a straight line, and from Kilbride to the flank of Reinn na Caillich 
above Coire-chat-achan — a direct distance of two miles and a quarter. A 
similar rock, possibly a portion of the same mass, appears in Creagan Dnbha, 
on the north side of the Red Hills. If the whole of this agglomerate forms 
part of one originally continuous mass, it must have been upwards of two 
miles in diameter. There may, however, have been two or three closely 
adjacent vents. The Beinn na Caillich patch, for example, appeals to 
belong to a different area, and that of Creagan Dubha is also probably 
distinct. Rut there seems no reason to doubt that the mass which forms 
Cnoc nam Fitlieach, and all the long declivity on the southern flank of 
Beinn Dearg Blieag, occupies part of the site of a single volcano. Owing 
to the absence of sufficient sections, it is hardly possible to determine how 
much of this fragmentary material should be assigned to the actual chimney. 
The diameter of the whole mass is almost two miles. But possibly a con- 
siderable proportion of this accumulation belongs to the external cone which 
gathered round the vent, so that the eruptive pipe might thus be of much 
smaller dimensions than the superficial area of the agglomerate. The sub- 
sequent invasion of so much granophyre, not only that ol the Red Hills, but 
that of numerous smaller intrusions, has indurated the agglomerate and 
made the investigation of its structure somewhat unsatisfactory. 
It might be supposed that the mere existence of intrusive bosses and 
veins rather furnishes an argument in favour of considering the visible 
agglomerate to belong to a deeper-seated part of the erupted material than 
the external cone. But, as will be afterwards shown, there is some reason to 
regard the present conical or dome-shaped outlines of the granophyre hills as 
not far from their original forms, and to believe that, like the traehytic Buys 
of Auvergne, they were much more superficial than plutonic eruptions. A 
study of the cinder cones of Central France shows that even these superficial 
accumulations have been invaded not only by bosses but by dykes. 1 
The agglomerate of the great Strath vent is a coarse tumultuous 
assemblage of blocks and bombs, imbedded in the usual dull, dirty-green 
matrix. Among the stones, grit and sandstone, together with scoriaceous, 
vesicular and amygdaloidal basalts are specially abundant ; also pieces of 
various quartz-porphyries and granophyres, among which a black telsite like 
that of Mull may often be recognised. In some places, large masses of 
altered limestone and quartzite (Cambrian) are included ; in others, pieces of 
yellow sandstone .and dark shale ( J urassic), or of the bedded lavas. Some of 
these masses may be 100 yards or more in length. Occasionally a breccia, 
mainly made up of acid materials — granophyre or granite,— has been noticed 
by Mr. Harker along the north side of the Red Hills, which he thinks may 
rather be of the nature of a crush-breccia than a part of the true agglomerate. 
The agglomerate of this district is wholly without stratification 01 
structure of any kind. On the north-west side of Loch Kilchrist, indeed, it 
1 The existence of a small dyke of andesite on the northern rim of the well-known crater of 
the Puy Parion has already been noticed. 
