CHAP. XXXV 
BRANCHING DYKES 
153 
Fia. 246. — Branching portion of the great Dyke near 
Hawick (length about one mile). 
come together again and enclose portions of the rocks through which they 
rise. But in reality such excrescences and bifurcations are of comparatively 
rare occurrence. As a rule, each dyke is a mere wall of igneous rock, with 
little more projection or ramification than may be seen in a stone field-fence. 
Among the short, narrow and 
irregular dykes of the gre- 
garious type branchings are 
occasionally seen, and in some 
districts are extraordinarily 
abundant. But among the 
great single dykes such irregu- 
larities are far less common 
than might have been looked 
for. A few characteristic 
examples from each type of 
dyke may here be given. 
The Cleveland dyke, which 
in so many respects is typical of the great solitary dykes of the 
country, has been traced for many miles without the appearance 
of a single offshoot of any kind. Yet here and there along its course, 
it departs from its usual regularity. As it crosses the Carboniferous 
tracts of Durham and Cumberland, there appear near its course lateral 
masses of eruptive rock, most of which doubtless belong to the much older 
“ Whin Sill.” But there is at least one locality, at Bolam near Cockfield, in 
the county of Durham, where the dyke, crossing the Millstone Grit, suddenly 
expands into a boss, and immediately contracts to its usual dimensions. 
Around this knot several short dykes or veins seem to radiate from it. The 
dyke has been quarried here, and its relations to the surrounding strata 
have been laid bare, as will be again referred to a little further on . 1 
Among the great persistent dykes of Scotland the absence of bifurcation 
and lateral offshoots offers a striking contrast to the behaviour of the dykes in 
those districts where they are small in size and many in number. But 
exceptions to the general rule may be gathered. Thus the Eskdale dyke is 
Fig. 247. — Branching Dyke at foot of Glen Artney (length about four miles). 
flanked at Wat Carrick with a large lateral vein, which is almost certainly 
connected with the main fissure. The Hawick and Cheviot dyke splits up 
on the hill immediately to the east of the town of Hawick, sends off some 
1 This locality was well described by Sedgwick, in liis early paper on Trap-Dykes in Yorkshire 
a nd Durham, Trans. Cambridge Phil. Soe. ii. p. 27. 
