148 
THE TERTIARY VOLCANOES 
BOOK VIII 
the high uplands of Cleveland. Its course across 
the ridges and valleys there has been carefully 
traced for the Geological Survey by Mr. G. Barrow, 
who has shown that over certain parts ol its course 
it does not reach the surface, but remains concealed 
under the Jurassic rocks, which it never succeeded 
in penetrating. But that in places it coru.es 
within a few feet of the soil is shown by the 
baked shale at the surface, for the alteration 
which it has induced on the surrounding rocks 
only extends a few feet from its margin. These 
interruptions of continuity show how uneven is 
the upper limit of the dyke. The characteristic 
porphyritic rock may he observed running up 
one side of a hill to the crest, but never reaching 
the surface on the other side. At Cliff Ridge, 
for example, about three miles south-west of 
Guisbrough, Mr. Barrow has followed it up to the 
summit on the west side ; but has found that on 
the east side it does not pierce the shales, which 
there form the declivity. This structure is 
represented in Fig 241. The vertical distance 
between the summit to the left, where the dyke 
( b ) disappears, and the point to the right, where 
the Lias shale (a) of the hill-side is concealed by 
drift (c), amounts to 250 feet, the horizontal 
distance being a little more than 900 feet. But 
as the shale when last seen at the foot of the 
slope is quite unaltered, the dyke must there be 
still some little distance beneath the surface, so 
that the vertical extension of this upward 
tongue of the dyke must be more than 250 feet. 
Mr. Barrow, to whom I am indebted for these 
particulars, has also drawn the accompanying 
section (Fig. 242) along the course of the dyke 
for a distance of nearly 11 miles eastward from 
the locality represented in Fig. 241. From this 
section it will be observed that in that space 
there are at least three tongues or upward pro- 
jections of the upper limit of the dyke. Several 
additional examples of the same structure are 
to be seen further east towards the last visible 
outcrop of the dyke. 
Another feature connected with the upward 
termination of the dyke is well seen in some parts 
of the ground through which the two foregoing 
