DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



55 



It will be observed that in these sections (figs. 7 and 10) there is 

 a curiously approximate coincidence between the inequalities in the 

 upper surface of the dyke and those in the form of the overlying 

 ground. The coincidence is too marked and too often repeated to be 

 merely accidental. Whether the ancient topographical features had 

 any influence in determining, by cooling or otherwise, the limit of the 

 upward rise of the lava, or whether the dyke, even though concealed, 

 has affected the progress of the denudation of the ground overlying 

 it, is a question worthy of fuller investigation. 



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§ 11. Known Vertical Extension. 



Closely connected with the determination of the upper limit reached 

 by the dykes, is the total vertical depth 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. In the dark gabbro hills of Skye, numerous dykes may 

 be seen climbing from the glens right up the steep rugged acclivities 

 and over the crests. In these and similar cases, we can actually trace 

 the dykes through a vertical thickness of more than 3000 feet of 

 rock. 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 pro- 

 bably extend over the district in full development ; and below them 

 there must lie a vast depth of Upper and Lower Silurian strata, 



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