Fig. 177.--Sections of the Carboniferous Limestone series of Northumberland showing the variations in the position of the Whin Sill. By Messrs. Topley and Lebour.] 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.[16] Limestone often shows only slight traces of change, though here and there it has become crystalline. [Footnote 16: Mr. Teall, _op. cit._ xxxix. (1884), p. 642, and authors cited by him.] 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 information 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 ejecting there both lavas and tuffs. It appears to be 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 be 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.[17] 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. [Footnote 17: See Messrs. Topley and Lebour, _op. cit._ p. 418.]