CHAP. XXIX 
THE GREA T WHIN SILL 
5 
series over an area of 1000 square miles we should probably be still below 
the truth. 
The rock composing this vast intrusive sheet is a dolerite or diabase, 
which maintains throughout its wide extent a remarkable uniformity of 
petrographical characters. In this and other respects it illustrates the 
typical features of sills. Thus it is coarsest in texture where it is thickest, 
and somewhat finer in grain towards its upper and lower surfaces than in 
the centre. Among the coarser varieties the component crystals of augite 
are not infrequently an inch in length and occur in irregular patches. 1 
Occasional amygdaloidal portions are observable, but these are not more 
marked than those to be found in the “ whin-dykes ” of the same region. 2 
the amygdaloidal and vesicular fine-grained rocks of the Bamborough district 
may possibly be quite distinct from the main body of the Whin Sill. 
Under the microscope the rock is seen to consist essentially of the 
usual minerals — plagioclase, augite and titaniferous magnetic iron-ore. An 
ophitic intergrowth of the augite and felspar is observable, likewise a certain 
quantity of micropegmatite which plays the part of groundmass between 
the interstices of the lath-shaped felspars. Full details of the characteristics 
of the component minerals and their arrangement are given by Mr. Teall in 
the paper already cited. 
I he main body of the sill is a sheet which sometimes diminishes to less 
than 20 feet in thickness and sometimes expands to 150 feet, but averages 
from 80 to 100 feet. It occasionally divides, as near Great Bavington, 
\\here it appears at the surface in two distinct beds separated by an 
intervening group of limestones and shales. Occasionally, as at Elf’s 
Hill Quarry, it gives out branches which send strings into the adjacent 
limestone. 3 
Although in most natural sections it seems to lie quite parallel with 
the strata above and below, yet a number of examples of its actual intrusion 
have been observed. When traced across the country, it is found not to 
remain on a definite horizon, but to pass transgressively across considerable 
thicknesses of strata. Its variations in this respect are well shown in the 
accompanying table of comparative sections constructed by Messrs. Topley 
and Lebour. 4 It will be seen that while at Harlow Hill the sill is found 
overlying the Great Limestone of Alston Moor, at liugley, five miles off it 
lies about 1000 feet lower down, far below the position of the Tyne-bottom 
Limestone. Still farther north, however, the sill west of Holy Island is 
said to lie 800 feet above the Great Limestone and to come among the 
higher beds of the Carboniferous Limestone series. 5 
The Whin Sill appears generally to thicken in an easterly or north- 
easterly direction. There are further indications that it was intruded from 
east to west. Thus, at Shepherd’s Gap, on the Great Homan Wall, the 
1 Sedgwick, Cambridge Phil. Trans, ii. p. 166. Mr. Teall, Quart. Journ. Oeol. Soc. xl. 
p. 643. 
Messrs. Topley and Lebour, Quart. Journ. Geol. Soc. xxxiii. p. 418. 
3 Messrs. Topley and Lebour, op. cit. p. 413. 
4 Op. cit. plate xviii. 5 Op. cit. p. 414. 
