THE TRAP DYKES OF THE ORKNEYS. 867 



yards. Without doubt it is in the West Mainland that they principally occur. In the 

 splendid coast section which stretches from Breckness, near Stromness, to the Brough 

 of Birsay, there is not a mile in which one cannot be found, and in several places there 

 are three, four, or six to a mile. On the east coast of the West Mainland also, in Evie, 

 Rendall, and Firth, although here the shores are low and gravelly, dykes are known to 

 be present in considerable numbers. From the East Mainland, on the other hand, 

 specimens of only two have been collected. The town of Kirkwall seems to stand on 

 the boundary-line between the region of dykes and that from which dykes are almost 

 absent. Five dykes were observed immediately to the west of the town, while none 

 was seen for ten miles to the east of it. 



Of the North Isles, Eousay is the only one which has yielded trap dykes; along the 

 western and southern shore they are very frequent, and bear a great resemblance to 

 those of the West Mainland. From the other islands they are not yet recorded, and 

 although I have examined the whole shore of the principal islands, I failed to obtain 

 any, so that they at least cannot be numerous. Of the South Isles I. cannot speak with 

 the same certainty, but in South Ronaldshay several are known which present interesting 

 features of their own. While there cannot be any doubt that in the future many 

 additions will be made to the numbers recorded, yet it is probable that the main facts 

 of their distribution are already established. 



Moreover, it is frequently to be observed that they tend to occur in groups, the 

 members of which are at no great distance from one another, and show a close resem- 

 blance in petrological character. One such group, for example, includes those of South 

 Ronaldshay. The dykes of Rousay, Birsay, and Evie form a well-marked class, closely 

 similar to one another. Those also which are exposed in the coast section just north of 

 the Black Craig on the west coast of the Mainland have certain common features, which 

 at the same time are fairly distinctive. In the little bay at Binniaro, Firth, eight dykes 

 occur, which in some points are quite unlike any other Orkney dykes. Instances might 

 be multiplied, but sufficient has been adduced to show that a tendency exists for similar 

 dykes to be grouped within a short distance of one another. It seems quite certain 

 that this is not due to the branching and subdivision of one main dyke, though that is 

 admittedly of common occurrence. In that case the distance which separates the 

 branch dykes is never great, and they are inconstant, frequently uniting within a short 

 distance only to diverge again, so that their true nature is quite obvious. It seems 

 rather that the dykes of one group were all of the same, or about the same age, while 

 those of other groups were injected at a different period. The common magma from 

 which they all proceeded was one which from time to time underwent considerable 

 change, and the fact that each group has common petrographical characters which differ 

 from those of other groups would indicate that each series was the product of a separate 

 phase in the cycle of change through which the magma passed. As will be seen 

 later, there is evidence of another kind that certain of these dykes are of later origin 

 than others. 



