CHAP. -XVIII 
VENTS 
289 
further to the south and west. More particular reference to this district 
will be made in the following chapter (p. 303). 
Vents which have been filled up with agglomerate, and which thus 
furnish the most obvious proofs of their connection with the eruptions ol 
the volcanic series, though not trequent, may be obserAed in a numbei ot 
the volcanic districts. Their fragmentary materials generally consist mainly 
of the detritus of andesites or diabases like those which lorm the liedded 
lavas. But where more acid lavas have risen to the surface, tiagmeiits oi 
felsite niay occur more or less abundantly. In the great vent of the Biaid 
Hills the tuffs and breccias are almost wholly acid. Hon-volcanic mateiials 
may often be found in the agglomerates, and occasionally even to the exclu- 
sion of volcanic detritus. Thus, in the far north of Scotland seveial 
examples occur among the Shetland Isles of necks filled entirely with 
blocks of the surrounding fiagstones and sandstones. Such eases, as has 
been already pointed out, probably represent incompleted volcanoes, wlien 
tlie explosive vapours were powerful enough to drill orifices in the crust ot 
the earth and eject the shattered debris from them, but were not sufficiently 
vigorous or lasting to bring up any solid or liquid A’olcanic material to the 
surface. These Shetland examples are further noticed on p. 345. 
jS ecks of agglomerate in the Lower Old Bed Sandstone vaij in size tioin 
a great orifice measuring two miles across to little plugs only a few } ards in 
diameter. They may be found in limited numbers in most of the volcanic 
districts. No examples have lieen observed rising through older rocks than 
the Old Eed Sandstone, all the known instances being eruptive thiAuigh 
some part of the volcanic series or of the sandstones, and therefore not 
belonging to the earliest eruptions. 
The largest, and in some respects the most interesting, vent in the 
Lowm- Old Bed Sandstone, that of the Braid Hills near Edinburgh, described 
in Chapter xx., covers an area of more than two square miles, and is filled 
with felsitic breccias and tuffs, through wdiich bosses and veins ot acid and 
basic rocks have been injected. It completely truncates the bedded lavas 
and tuffs of the Bentlaud Hills, and not improbably marks the chief centre 
from which these rocks wnre erupted. Several smaller necks rise a little 
beyond its southern margin, marking, perhaps, lateral cones on the mam 
volcano. 
In the small area of Lower Old Bed Sandstone lying between Campbel- 
town and the Mull of Cantyre, several necks of agglomerate occur, which 
have been partly dissected bv the waves along the shore, thus revealing their 
internal structure and their ‘'relation to the surrounding conglomerates. An 
account of them wdll be found at p. 311. One of the series, which lies back 
from the coast-line, forms a prominent rounded hill measuring about 400 
yards in its longest diameter. Its general contour is represented in 
Fig. 82. 
Of the eruptive bosses of massive rock outside the limits of the Old Bed 
Sandstone which may be plausibly referred to the volcanic phenomena of the 
period, though they cannot be proved to be actually part of them, the most 
