88 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
with the luechaiiisni of the remaining intrusive masses which have now to 
be described. 
iii. Bosses {Stocks, Culots) 
The term Boss has been applied to masses of intrusive rock which form 
at the surface rounded, craggy or variously -shaped eminences, having a 
circular, elliptical or irregular ground-plan, and descending into the terrestrial 
crust with vertical or steeply-inclined sides (Kg. 28 ). Sometimes they can 
be seen to have pushed the surrounding rocks aside. In other places they 
seem to occupy the place of these rocks through which, as it were, an 
opening has been punched for the reception of the intrusive material. 
Occasionally, more especially in the case of large bosses, like those in 
which granite so frequently appears, the eruptive mass may he observe<l to 
rise here and there in detached knobs through the surrounding rocks, or to 
enclose patches of these, in such a manner as to indicate that the large body 
of eruptive material terminates upward in a very irregular surface, of which 
only the more prominent parts project through the cake of ovei’lying rocks. 
In true bosses, luilike sills or laccolites, we do not get to any bottom on 
which the eruptive material rests. Laccolites, indeed, may he regarded as 
intermediate between the typical sill and the typical boss. Ilie difference 
between a laccolite and a boss lies in the fact that the body of the laccolite 
does not deseeird into an unknown depth in the crust, hut lies upon a 
platform on which it has accumulated, the magma having ascended by one 
or more duets, which generally hear hut a small proportion in area to the 
mass of the laccolite. The boss, on the other hand, is not known to lie on 
any horizon, nor to ijroceed from smaller ducts underneath, but plunges as a 
great pillar or irregular mass, which may frequently he noticed to widen down- 
wards into the crust. There can be no doubt, however, that many masses of 
eruptive rock, which, according to the definition here given, should be called 
bosses, would be found to be truly laccolites if their structure below ground 
could he ascertained. It is obvious that our failure to find any platform 
on which the body of a boss lies, may arise merely from denudation having 
been as yet insufficient to lay such a platform l)are. It is hardly probable 
that a boss several miles in diameter should descend as a column of that 
magnitude to the magma-reservoir from which its material came. More 
probably it has been supplied through one or more smaller ducts. The large 
boss now visible at the surface may thus be really a laccolitie expansion on 
one or more horizons. M. Michel Ldvy lays stress on the general widening 
of granitic bosses as they descend into the crust.^ AVhile his observations 
are supported by many illustrations from all parts of the globe, and 
are probably true of the deeper-seated masses of granite, it is no less 
true that numerous examples have been met with where a granite boss is 
sharply marked off from the rocks which it has invaded and on which it may 
' M. Michel Levy, Bull. Carte GeoJ. France, No. 35, touio v. (1893), p. 32. The view stated 
in tlie text is also that adopted hy Prof. Briigger with reference to the granite of the Christiania dis- 
trict. “Die Krnptivgesteine des Kristianiagebietes.” 
