CHAP. VI 
SILLS AND LACCOLITES 
83 
greater alteration of the same rocks by some dykes than by others, a sign of 
such a connection with the surface may survive. This subject will be again 
referred to in the account of the Tertiary dykes of Britain in Book VIII., 
where the whole of the phenomena of this phase of volcanic action will be 
fully discussed (see vol. ii. p. 163). 
ii. Sills and Laccolites 
The word “ sill,” derived from a remarkable sheet of eruptive rock in the 
north of England, known as the Great Whin Sill (Chapter xxix.), is now 
applied as a convenient general term to masses of intrusive material, which 
have been injected between such divisional planes as those of stratification, 
and which now appear as sheets or beds (Fig. 33). These masses are like- 
wise called Intrusive Sheets, and where the injected material has accumulated 
in large blister-like expansions, these are known as Laccolites (Fig. 34). 
Sills vary from only an inch or two up to 5 0 0 feet or more in thickness. 
Lying, as they frequently do, parallel with strata above and below them, 
they resemble in some respects true lava-sheets erupted contemporaneously 
Fig. 33. — Section of Sill or Intrusive Sheet. 
with the series of sediments among which they are intercalated. And, 
indeed, cases occui' in which it is hardly possible to decide whether to regard 
a given mass as a sill or as a superficial lava. In general, however, sills 
exhibit the coarser texture above referred to as specially characteristic of 
subterranean eruptive masses. Moreover they are usually, though not 
always, free from the vesicular and amygdaloidal structures of true surface- 
lavas. Their under and upper surfaces, unlike the more scoriaceous parts of 
lavas, are commonly much closer in grain than the general body of the 
mass ; in other words, they possess chilled borders, the result of more rapid 
consolidation by contact with cooler rock. Again, instead of conforming to 
the stratification of the formations among which they lie, as truly inter- 
stratified lavas do, they may be seen to break across the bedding and pursue 
their course on a higher or lower jilatforni. The strata that overlie them, 
instead of enclosing pieces of them and wrapping round irregularities on 
their surface, as in the case of contemporaneously erupted lava-sheets, are 
usually indurated, sometimes even considerably altered, while in many cases 
they are invaded by veins from the eruptive sheet, or portions of them are 
involved in it, and are then much hardened or metamorphosed. 
The petrographical character of the sills in a volcanic district depends 
