38 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Fig. 2.— Arrangement of lines of Amygdules 

 in a Dyke, Strathmore, Skye. 



presence of amygdules. It has sometimes been supposed that the amygdaloidal structure 

 may be relied upon as a test to distinguish a mass of molten rock which has reached the 

 surface, from one which has consolidated under considerable pressure below ground. 

 That this supposition, however, is erroneous, is demonstrated by hundreds of dykes in the 

 great system which I am now describing. But the amygdules of a dyke offer certain 

 peculiarities which serve in a general way to mark them off from those of an outflowing 

 lava. They are usually smaller, and more uniform in size, than in the latter rock. They 

 are also more regularly spherical and less frequently elongated in the direction of flow. 

 Moreover, they arc not usually distributed through the whole breadth of a dyke, but 



tend to arrange themselves in lines especially 

 towards its centre (fig. 2). In these central 

 bands the cavities are largest and depart farthest 

 from the regular spherical form, so that for short 

 spaces they may equal in bulk the mass of en- 

 closing rock. In some rare instances, a whole 

 dyke is composed of cellular basalt, like one of 

 the sheets in the plateaux, as may be seen on 

 the north flank of Beinn Suardal, Skye. 



Besides the common arrangement of fine- 

 grained edges and a more coarsely crystalline 

 centre, instances are found where one of the 

 contrasted portions of a dyke traverses the other in the form of veins. Of these, I think, 

 there are two distinct kinds, probably originating in entirely different conditions. In 

 the first place, they may be of coarser grain than the rest of the rock; but such a 

 structure appears to be of extremely rare occurrence. I have noticed some examples 

 on the coast of Renfrewshire, where strings of a more coarsely crystalline texture 

 traverse the finer-grained body of the rock. Veins of this kind are probably of the 

 same nature as the segregation- veins, to be afterwards referred to as a frequent 

 occurrence among the thicker intrusive sheets. They consist of the same minerals 

 as the rest of the rock, but in a different and more developed crystalline arrangement, 

 and they contain no glassy or devitrified material, except such portions of that of 

 the surrounding ground-mass as may have been caught between their crystalline 

 constituents. 



The second kind of veins, which though not common, is of much more frequent occur- 

 rence than the first, is more particularly to be met with among the broader dykes, and 

 is distinguished by a remarkable fineness of grain, sometimes approaching the texture of 

 felsite or jasper, and occasionally taking the form of actual glass. Such veins vary from 

 half an inch or less, up to four or five inches in breadth. They run sometimes parallel 

 with the walls of the dyke, but often irregularly in all directions, and for the most part 

 avoid the marginal portions, though now and then coming up to the edge. They never 

 extend beyond the body of the dyke itself into the surrounding rock. Though they have 



