CHAP. VI 
VOLCANIC BOSSES 
93 
vitles him with some data for an elucidation of the cause of the sequence of 
erupted products during a volcanic cycle. In the second place, it yields to 
him some interesting analogies with the structures of ancient gneisses, and 
tlius helps towards the comprehension of the origin and history of these 
profoundly difficult hut deeply fascinating rocks. 
Bosses, like sills, occur in the midst of volcanic slieets, and also as soli- 
tary protrusions. AVhere they rise amidst interstratified lavas and tuffs they 
may often be recognized as occupying the position of volcanic vents. They 
are then necks, and their characters in this connection have already been 
given. Where, however, as so frecpiently happens, they appear among rocks 
in which no trace of any contemporaneous volcanic material is to be detected, 
their relation to former volcanic activity remains uncertain. 
Of this doubtful nature some of the most notable e.xamples are supplied 
by the great granitic bosses which occur so frequently among the older 
Baheozoic rocks of Britain. The age of these can sometimes be approximately 
fixed, and is then found to correspond more or less closely with some vol- 
canic episode. Tims the granite-bosses of Galloway, in the south of Scotland, 
disrupt Upper Silurian strata, but are older than the Upper Old Sandstone. 
Hence they probably belong to the period of the Lower Old Eed Sandstone, 
which was eminently characterized by the vigour and long continuance of 
its volcanoes. The granite of Arran and of the Mourne Mountains can be 
shown by one line of reasoning to be younger than surrounding Carboniferous 
formations, by other arguments to he probably later than the Permian period, 
and by a review of the whole evidence to form almost certainly part of the 
volcanic history of Tertiary time. 
But even where it can be shown that the uprise of a huge boss of 
eruptive material was geologically contemporaneous with energetic volcanic 
action, this coincidence may not warrant the conclusion that the boss there- 
fore marks one of the volcanic centres of activity. Each example must be 
judged by itself There have, doubtless, been many cases of the intrusion 
of molten material in bosses, as well as in sills, without the establishment 
of any connection with the surface. Such incompleted volcanoes have been 
revealed by denudation after the removal of a great thickness of superin- 
cumbent rock. The evidence which would have decided the question to 
what extent tuny of them became true volcanic vents has thus been destroyed. 
We can only reason tentatively from a careful collation of all the facts that 
are now recoverable. Illustrations of this kind of reasoning will be fully 
given in subsequent chapters. 
It has been supposed that a test for the discrimination of a subterranean 
protrusion from a true volcanic chimney may be found in the condition of 
the surrounding rocks, which in the case of the prolonged flow of molten 
matter up a vent would be likely to undergo far more metamorphism than 
would be the case in the injection of a single eruptive mass.^ But, as 
has been already pointed out, no special or excessive metamorphism of the 
encircling rocks is noticeable around many vents. There is certainly no 
^ See, for example, Mr. Harker, Quart. Jowni. trcol. Soc. 1. (1894), p. 329. 
