THE TERTIARY VOLCANOES 
BOOK VIII 
154 
branches, and then resumes its normal course (Fig. 246). Again, one of the 
two nearly parallel dykes which run from Loehgoilhead across Ben Ledi 
into Glen Artney bifurcates at the foot of that valley, its northern limb 
(about two miles long) speedily dying out, and its southern branch throwing 
off another lateral vein, and then continuing eastward as the main dyke 
(Fig. 247). 
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 become familiar to geologists from the 
sketches given by Macculloch from the lower parts of the cliffs of Trotternish 
in Skye. 1 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 casital tourists in the passing steamboats. The veins vary 
in width from less than an inch to several feet or yards. They run in all 
directions and intersect each other, forming such a confused medley as 
requires some patience on the part of the geologist who would follow out 
each independent ribbon of injected material in its course up the cliffs, or 
still more, would sketch their ramifications in his note-book. A good, 
though perhaps somewhat exaggerated, illustration of their general character 
was given by Macculloch. 2 The accompanying figure (Fig. 248) is less 
sensational, but represents with as much accuracy as I could reach, the net- 
work of veins near the foot of the cliffs. One conspicuous group of veins, 
which, seen from a distance, looks like a rude sketch of a lug-sail traced in 
black outline upon a pale ground, is known to the boatmen as “ M'Eiven’s 
Sail.” Another admirable locality for the study of dykes and tortuous veins 
is the northern coast of the Sound of Soa, where an extraordinary number 
of injections traverse the Torridon Sandstones on which the plateau-basalts 
rest (Fig. 323). 
As a general rule, the narrower the vein the finer in grain is the rock 
of which it consists. This compact dark homogeneous material has 
commonly passed by the name of “ basalt.” Its minuteness of texture prob- 
ably in most cases arises from local rapidity of cooling, and it is doubtless 
tlie same substance which, where in larger mass in the immediate neigh- 
bourhood, has solidified as one of the other pyroxene-plagioclase-magnetite 
rocks. 
With regard to the places where such abundant tortuous veins are more 
especially developed, I may remark that they are particularly prominent 
under a thick overlying mass of erupted rock, such as a great intrusive 
sheet, or the bedded basalts of the plateaux, or where there is good reason 
1 Western Islands, plate xvii. 2 Op. cit. plate xxxiii. Fig. 1. 
