CHAP. XXIX 
THE GREA T WHIN SILL 
7 
Weardale the “Little Whin Sill” diminishes from 20 feet, till in three 
miles it disappears. 1 
The strata in contact with the Whin Sill, both above and below, have 
been more or less altered. Sandstones have been least affected ; shales 
have suffered most, passing into a kind of porcellanite, with development of 
garnet and other minerals. 2 Limestone often shows only slight traces of 
change, though here and there it has become crystalline. 
No trace of any boss or neck has been detected in the whole region 
which might be supposed to mark a funnel of ascent for the material of 
the Whin Sill. The Hett Dyke and the High Green Dyke, already noticed, 
may, however, have been possibly connected with the injection of this great 
intrusive sheet. No other visible mass of igneous rock in the region has 
been even plausibly conjectured to indicate a point or line of emission for 
the sill. 
It is certainly singular that in so wide a territory, where the whole 
succession of strata has been so admirably laid bare by denudation in 
thousands of natural sections, and where, moreover, much additional informa- 
tion has been obtained from lead-mining as to the nature of the rocks below 
ground, not a single vestige of tuff, agglomerate or interstratified lava has 
been up to the present time recorded, unless the Harkess rocks already 
alluded to can be so regarded. 
Judging, however, from the analogy of the other districts of igneous 
rocks in Britain, we can hardly resist the conclusion that the Great Whin 
Sill is essentially a manifestation of volcanic action, that it was connected 
with the uprise of basic lava in volcanic orifices, and that the subterranean 
energy may quite probably have succeeded in reaching the surface and eject- 
ing there both lavas and tuffs. 
It appears to he certain that any vents which existed cannot have lain 
to the west of the present escarpment of the sill, for no trace of them can 
he found there piercing the Carboniferous or older formations. They 
must have lain somewhere to the east in the area now overspread with 
Millstone Grit and Coal-measures, or still farther east in the tract now 
concealed under the North Sea. The evidence of the sill itself, as we have 
seen, corroborates this view of the probable situation of the centre of 
disturbance. 
The question of the geological age of the sill is one of considerable 
difficulty, to which no confident answer can be given. 8 The injection of the 
diabase must obviously be considerably later than the highest strata through 
which it has risen ; that is, it must be younger than some of the higher 
members of the Carboniferous Limestone series. But here our positive 
evidence fails. 
The Sill is traversed by the same faults which disrupt the surround- 
ing Carboniferous rocks. It is therefore of older date than these 
dislocations. Its striking general parallelism with the shales and lime- 
1 Op. cit. p. 419. 2 Mr. Teall, op. cit. xxxix. (1884), p. 642, and autliors cited by him. 
3 See Messrs. Topley and Lebour, op. cit . p. 418. 
