58 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



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.* The accompanying figure (fig. 13) is less 

 sensational, but represents with as much accuracy as I could reach, the network 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'Niven's Sail." 



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 



Fig. 13. — Basic Veins traversing Secondary Limestone and Sandstone on the coast cliffs, Ardnamurchan. 



of " basalt," but its minuteness of texture probably in most cases arises from local 

 rapidity of cooling, and it may be the same substance which, where in larger mass in the 

 immediate neighbourhood, 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 to believe that such a deep cover, though now removed by 

 denudation, once overspread the area in which they appear. It will be shown in the 

 sequel that such horizons have been peculiarly liable to intrusions of igneous material of 

 various kinds, and at many different intervals, during the volcanic period. A thick cake 



* Op. cit., plate xxxiii. fig. 1. 



