CHAP. XXIX 
THE GREA T WHIN SILL 
3 
I>ut not until he has traced these ridges for many miles southwards and 
tound their component rocks to form there an almost continuous sheet does 
lie realize that nothing of the kind among the Scottish Carboniferous rocks 
can be compared for extent to this display in the North of England. 1 
biom the furthest skerries of the Fame Islands southwards to Burton 
ell on the great Pennine escarpment, a distance in a straight line of about 
80 miles, this intrusive sheet may be traced in the Carboniferous Limestone 
senes (Map I.). There are intervals where its continuity cannot he actually 
o lowed at the surface, but that it really runs unbroken from one end to the 
Ti ■' l Under § r0lind caun °f ^ )G doubted by any one who has examined the region, 
lhis singular feature in the geology and scenery of the North of England is 
viiown locally as the Great Whin Sill. 2 From the rocky islets and castle- 
ciowned ciags of the coast-line it maintains its characteristic topography, 
structure and composition throughout its long course in the interior. So 
regularly parallel with the sedimentary strata does it appear to lie, that it 
was formerly regarded by many observers as a true lava-sheet, poured out 
upon the sea-floor over which the limestones and shales were laid down 
But its really intrusive character has now been clearly demonstrated. Not 
a vestige of any tuff has been detected associated with it, nor does it ever 
present the usual characters of a true lava-stream. 3 Its internal structure 
and the wonderful uniformity in its character mark it out as a typical 
intrusive sheet. ' 1 
mong the manifestations of the subterranean intrusion of igneous rocks 
m the British Isles the Great Whin Sill, next after the Dalradian sills of 
Scotland, is the most extensive. Its striking continuity for so great 
c distance, and the absence around it of any other trace of igneous action 
save a few dykes, place it in marked contrast to the ordinary type of 
arbomferous sills. The occasional gaps on its line of outcrop in the 
hasbSn dliftn £ bee " f 16 + . UbjeC l° f mU ° h disCUSsi0n - and a S° od ** of geological literature 
and Tate C ° n8lde . ratl0n - Tll « writings of Trevelyan, Sedgwick, W. Hutton, Phillips 
by Lme Of the e i P w m t % T^' The intrUsivo of the Sill, maintained 
-Hsculd and lln r’lT I*! 7 estilblisIled ^ mapping of tin, Geological Survey, and was 
o the 0 Jw r str t d , by Jlcssrs - W - T °l )le y “d G. A. Lebour in a paper in the 33rd volume 
See also P ^ r T ?°°n fr ' “ whicU raferonces to tire earlier observers will be found. 
, Outlines of the Geology of Northumberland, 2nd edit. (1886), p. 92. The 
pet Ography of the Whin Sill is fully treated by Mr. Teall in Quart. Journ. Geol. SoL «L (1884), 
p. 640, where ^ bibliography of the subject is also given. 
stone Whm n 1S a , oonlmon tmn in Scotland and the North of England for any hard kind of 
or h!d TT Y SU t “ Can . bu l,Sed f0r makin S and mending roads. “Sill” denotes a Hat course 
nn ... ° 3 *°. r ' e ’ ari< , was evidently applied to this intrusive sheet from its persistent flat-bedded 
position and its prominence among the other gently inclined strata among which it lies. It is from 
this example m the North of England that the word “ sill ” has passed into geological literature, 
the wr tbe ,®° ast at ^“thorough aud the Harkess Rocks the usual petrographioal characters of 
sheet - ^ ■! at ° e , XCbaIlged f0r tll08e of bne-gi’aiued amygdaloidal diabases arranged in distinct 
S ’ W f 111 1 r U,>per 11arts are b’Sb'y vusi «ular and show ropy surfaces— peculiarities 
,. §8 estIve ot true lava-streams. But according to Professor Lebour the rocks are intrusive into 
uies tone and shale ( Geology of Northumberland and Durham, p. 9S). Mr. Teall has expressed 
ie suspicion that these rocks must have consolidated under conditions somewhat different from 
those which characterized the normal Whin Sill (Quart. Journ. Geol. Soc. xl. p. 643). They 
oiitil G 0U ^ •P ar ^ 8 ^ le Present features that might possibly indicate superficial 
