CHAP. VI 
VOLCANIC BOSSES 
89 
be seen to lie. Apart from the cases where granite seems to form part of a 
vast internal, once molten mass, into which its encircling gneisses seem 
to graduate, there are others in which this rock, as now \daible, has been 
injected into the crust as a boss or as a laccolite. Instances will be described 
in later chapters where such bosses have risen through Cambi’ian, Silurian, 
Devonian and Carboniferous formations. It may be said that between such 
granitic intrusions and volcanic operations no connection can be traced. But 
reasons will be brought forward in later chapters to regard some of the 
granitic bosses as parts of the mechanism of Paheozoic volcanoes. It will 
also be shown tliat among the intrusive rocks of the Tertiary volcanic series 
of Britain there occur bosses of truly gTanophyric and granitic material. 
Hence, tliough mainly what is called a “ plutonic ” rock, granite has made 
its ap]5earance among the subterranean protrusions of volcanoes. 
It is no doubt true that many intrusive masses, which must be included 
under the general name of bosse.s, liave proliably had no connection what- 
ever with volcanic action properly so called. They are plutonic injections, 
that is, portions of the subterranean magma which have been intruded into 
the terrestrial crust during its periods of disturbance, and have not been 
accompanied with any superficial discharges, which are essential in truly 
volcanic energy. It has been proposed to draw a distinction between such 
deep-seated intrusions and those which represent volcanic funnels.’^ If this 
were always practicable it would certainly be desirable. But the distinction is 
not one that can in every case be satisfactorily drawn. Even in regard to 
granitic bosses, which may generally be assumed to be plutonic in origin, 
the Bi'itish examples just referred to have in all likelihood been connected 
with undoubted volcarrie outbursts. Without, therefore, attempting here to 
separate the obviously Amlcanic necks of eruptive material from the probably 
plutonic bosses, I propose to describe brielly the general characters ot bosses 
considered as a group of intrusive rocks, together with the phenomena 
which accompany them, and the conditions under which they may have 
been injected. 
Bosses, whether of plutonic or volcanic origin, are frequently not merely 
single masses of eruptive rock, but are accompanied with a system of dykes 
and veins, some of which can be traced directly into the parent-mass, while 
others traverse it as well as the surrounding rocks. Hence the history of a 
boss may be considerably more complex than the external form of the mass 
might suggest. 
The petrographical characters of bosses link them with the other under- 
ground injections of igneous material, more especially with sills and laccolites. 
Indeed, on mere lithological grounds no satisfactory line could be drawn 
between these various forms of intrusive rocks. The larger the mass the 
more coarsely crystalline it may be expected to be. But the whole range 
of structure, texture and composition, from those of the narrowest vein to 
those of the widest boss, constitutes one connected series of gradations. 
Acid, intermediate and basic rocks are abundantly displayed among the 
^ M. Micliel Ldvj’’, Bitll. Carte G^ol. P}'ance, No. 35, tome v. (1893). 
