CHAP. VI 
BOSSES AND SILLS 
97 
It is not easy to eoiupreliend the conditions under which large masses 
of molten material have been injected into the crust of the earth. The two 
main factors in volcanic action — terrestrial contraction and the energy of the 
vapours in the magma — have no doubt played the chief part in the process. 
But the relative share of each and the way in which the enormous load of 
overlying rock has been overcome are not readily intelligible. 
Let us first consider for a moment the pressure of the superincumbent 
crust under which the injection in many cases took place. The Whin Sill 
of England may serve as a good illustration of the difficulties of the problem. 
This notable mass of intrusive rock has been forced between the stratification 
planes of the Carboniferous Limestone series in one, or sometimes more than 
one, sheet. It stretches for a horizontal distance of not less than 80 miles 
with an average thickness of between 80 and 100 feet. Erom the area 
over which it can be traced its total extent underground must be at least 
400 scpiare miles (see Chapter xxix.). 
In any single section the Whin Sill might be supposed to be a truly 
iuterstratified sheet, so evenly does it seem to be intercalated between the 
sedimentary strata. But here and there it diverges upward or downward 
in such a way as to prove it to be really a vast injected sheet. The age of 
the injection cannot be precisely fixed. It must be later than the Carbon- 
iferous Limestone. There is no trace of any stratigraphical break in the 
Carboniferous system of the region traversed by the sill. If the injection 
took place during the Carboniferous period, it does not appear to have been 
attended with any local disturbance, such as we might suppose would have 
been likely to accompany the extravasation of so enormous a mass ot igneous 
material. If the date of injection be assigned to the next volcanic episode 
in the geological history of Britain — that of the I’ermian period — it will 
follow that the Whin Sill was intruded into its present position under the 
superincumbent weight of the whole of the Carboniferous system higher 
than the platform followed by the injected rock. The overlying body of 
strata w'ould thus exceed 5000 feet in thickness, or in round numbers would 
amount at least to an English mile. The pressure of this mass of super- 
incumbent material, at the depth at which the injected magma was forced 
between the strata, must have been so gigantic that it is difficult to believe 
that the energy of the magma would have been able to achieve of itself so 
stupendous a task as the formation of the Great Whin Sill. 
The volume of injected material is likewise deserving of special attention. 
Many sills exceed dOO or 400 feet in thickness; and .some laccolites must 
enormously surpass these limits. The intrusion of so vast a body of new 
material into the terrestrial crust will necessitate eitlier a corresponding eleva- 
tion of that part of the crust overlying the injected magma or a subsidence 
of that part underlying it, or some combination of both movements. It is 
conceivable that, where the body of protruded magma was large and the 
thickness of overlying crust was small, the expansive force of the vapours 
under high ten.sion in the molten rock may have sufficed for the uplilt. 
This result will be most likely to be effected around a volcanic chimney 
VOL. I H 
