CHAP. VI 
S/LLS AND LACCOLITES 
85 
structure is not due to successive injections of material among already 
consolidated rocks, but belongs to the original conditions of expulsion of the 
gabbro as a whole. It seems to indicate that the magma which supplied 
the sills was at the time of its extrusion heterogeneous in composition, and 
that the banding arises from the simultaneous or rapidly successive protrusion 
of different portions of this variously-constituted magma. The details of 
the structure will be described in the general account to be given of the 
Tertiary volcanic rocks (Chapters xliii. and xliv.). 
Besides such visible differences in the composition of sills, others much 
less obtrusive may occasionally be detected with the aid of microscopic or 
chemical research. The outer parts of some sills are thus discovered to be 
more basic or more acid than the inner portions. Or evidence may be 
obtained pointing to the probable melting down of surrounding rocks by 
the erupted magma, with a consequent local change in the chemical and 
mineralogical constitution of the mass. 
In regard to their position in the geological structure of an old volcanic 
district I may here remark that sills, seldom entirely absent, are more 
especially developed either among the rocks through which the volcano has 
driven its vent, or about the base of the erupted lavas and tuffs. Many 
illustrations of this distribution will be described from the various volcanic 
areas of Britain belonging to Palfeozoic and Tertiary time. At the base of 
the great Cambrian and Lower Silurian volcanic series of Merionethshire, 
sills are admirably developed, while among the basaltic eruptions which closed 
the long volcanic record in the north of Ireland and the Inner Hebrides, 
they play a notable part. 
From the frequent place which sills take at the base of a volcanic series, 
it may be inferred that they generally belong to a late phase in the history 
of an eruptive episode or cycle, when the orifices of discharge had become 
choked up, and when the volcanic energy found an easier passage laterally 
between the strata underneath the volcanic pile or between the sheets of 
that pile itself, than upward through the ever-increasing thickness of ejected 
material. 
While there is an obvious relation between most sills and some eruptive 
centre in their neighbourhood, cases occur in which no trace of any contem- 
poraneous volcano can be found, but where the intrusive sheet remains as the 
sole evidence of the movements of the subterranean magma. The Great 
Whin Sill, one of the most extensive intrusive sheets in the British Isles, is an 
instance of this kind. Though this large mass of injected material can be 
traced for a distance of about 80 miles, and though the strata beneath and 
above it are well exposed in innumerable sections, no evidence has yet been 
detected to show that it was connected with any vent that formed a volcano 
at the surface (see vol. ii. p. 2). The absence of this evidence may, of 
course, arise from the failure of denudation to uncover the site of the vent, 
which may possibly still remain buried under the Carboniferous strata that 
overlie the sill towards the south-east. But it may be due to the non-exist- 
ence of any such vent. We can quite conceive that volcanic energy should 
