DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 57 



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. # 



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 flanked at West 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 

 branches, and then resumes its normal course (fig. 11). Again, one of the two nearly 

 parallel dykes which run from Loch Goil Head across Ben Ledi into Glen Artney 

 bifurcates at the foot of that valley, its northern limb (about two miles long) speedily 



Fig. 11. — Branching portion of the great Dyke near Fig. 12. — Branching Dyke at foot of Glen Artney (length 



Hawick (length about 1 mile). about 4 miles). 



dying out, and its southern branch throwing off another lateral vein, and then continuing 

 eastward as the main dyke (fig. 12). 



In the districts of gregarious dykes, however, abundant instances may be found of 

 dykes that branch, and of others that lose the parallelism of their walls, become irregular 

 in breadth, direction, and inclination, so as to pass into those intrusive forms that are 

 more properly classed as veins. Excellent illustrations of bifurcating dykes may be 

 observed along the shores of the Firth of Clyde, particularly on the eastern coast line of 

 the isle of Arran. The venous character has been familiar to geologists from the sketches 

 given by Macculloch from the lower parts of the cliffs of Trotternish in Skye.t But 

 still more striking examples are to be seen in the breaker-beaten cliffs of Ardnamurchan. 

 The pale Secondary limestones and calcareous sandstones of that locality are traversed by 

 a series of dark basic veins, and the contrast of tint between the two kinds of rock is 

 so marked as even to catch the eye of casual tourists in the passing steamboats. The 



* This locality was well described by Sedgwick, in his early paper on Trap-Dykes in Yorkshire and Durham, Trans. 

 Cambridge Phil. Soc, ii. p. 27. 

 t Western Islands, plate xvii. 



