Closely connected with the determination of the upper limit reached by the dykes, is the total vertical distance to which they can be traced. Of course, the depth of the original reservoir of molten rock which supplied them remains unknown, and probably undiscoverable. But it is possible, in many cases, to determine at least the inferior limit of the thickness of rock through which the molten material of the dykes has ascended. Along the great basalt-escarpments of Mull and Skye, the ascent of dykes from base to summit may often be observed. Thus, on the cliffs of Dunvegan Head, on the west coast of Skye, which rise out of the sea to a height of about 1000 feet, several dykes may be observed rising through the whole series of basalts up to the crest of the precipice. In the dark gabbro hills of the same island, numerous dykes may be seen climbing from the glens right up the steep rugged acclivities and over the crests, through a vertical thickness of more than 3000 feet of rock (Fig. 333). The dykes which cross Loch Lomond, and ascend the hills on either side of that deep depression, must rise through at least as great a thickness. But where a knowledge of the geological structure of the ground enables us to estimate the bulk of the successive rock-formations which underlie the surface, it can be shown that the lava ascended through a much greater depth of rock. Measurements of this kind can best be made towards the eastern end of the Cleveland dyke, where the different sedimentary groups have not been seriously disturbed, and where, from natural sections and artificial borings, their thicknesses are capable of satisfactory computation. The highest bed of the Jurassic series anywhere touched by the dyke is the Cornbrash. It is certain, therefore, that the igneous rock rises through all the subjacent members of the Jurassic series up to that horizon. There can be no doubt also that the Trias and Magnesian Limestone continue in their normal thickness underneath the Jurassic strata. To what extent the Coal-measures exist under Cleveland has not been ascertained; possibly they have been entirely denuded from that area, as from the ground to the west. But the Millstone Grit and Carboniferous Limestone probably extend over the district in full development; and below them there must lie a vast depth of Upper and Lower Silurian strata, probably also of still older Palæozoic rocks and beneath all the thick Archæan platform. Tabulating these successive geological formations, and taking only the ascertained thickness of each in the district, we find that they give the results shown in the subjoined table.[190] [Footnote 190: Drawn up for me by Mr. G. Barrow.] STRATA CUT BY THE CLEVELAND DYKE