THE TERTIARY VOLCANOES 
BOOK VIII 
distance of five miles, as shown in Fig. 245, which has been 
drawn for me by Mr. J. G-. Goodchild. 
It will be observed that in these sections (Figs. 241, 
242 and 245) there is a curiously approximate coincidence 
between the inequalities in the tipper 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. 
11. KNOWN VERTICAL EXTENSION 
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 
