DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 51 



on the whole maintained. In such cases, it is evident that the fissures were not long 

 straight dislocations like the larger lines of fault in the earth's crust, but were rather 

 notched rents or cracks which, though keeping on the whole one dominant direction, were 

 continually being deflected for short distances to either side. As a good illustration of 

 this character, reference may be made to the Cheviot and Hawick dyke. In Teviotdale 

 this dyke can be followed continuously among the rocky knolls, so that its deviations 

 can be seen and mapped. From the median line of average trend the salient angles 

 sometimes retire fully a quarter of a mile on either side. Some examples of the same 

 feature may be noticed in the Eskdale dyke. The large dyke which runs westward from 

 Dunoon has been observed by Mr Clough to change sharply in direction three times in 

 four miles, running sometimes for a short distance at a right angle to its general 

 direction (see fig. 17). 



Among these solitary dykes also, though the persistence of their trend is so pre- 

 dominant, there occur instances where the general direction undergoes great change. 

 Some of the most remarkable cases of this kind have been mapped by Mr B. N. Peach 

 and Mr R. L. Jack, in the course of the Geological Survey of Perthshire. Several 

 important dykes strike across the Old Red Sandstone plain for many miles in a direction 

 slightly south of west. But when they approach the rocks of the Highland border in 

 Glen Artney, they bend round to south-west, and continue their course along that new line. 



In my early paper on the " Chronology of the Trap-Rocks of Scotland," * I called 

 attention to the dominant trend of the dykes from N.W. to S.E. Subsequent 

 research has shown this to be on the whole the prevalent direction throughout the 

 whole region of dykes. But the detailed mapping, carried on by my colleagues and 

 myself in the Geological Survey, has brought to light some curious and interesting 

 variations from the normal trend. In the districts where dykes of the gregarious type 

 abound there is sometimes no one prevalent direction, but the dykes strike to almost 

 all points of the compass. Of the Arran dykes, so carefully catalogued by Necker, only 

 about a third have a general north-westerly course. But in Eastern Argyleshire the 

 abundant dykes mapped by Mr Clough trend almost without exception towards N.N.W. 

 In the north of Ireland Berger found the direction of thirty-one dykes to vary from 

 17° to 71° W. of N., giving a mean of N. 36° W.t In Islay, Jura, Eigg, Mull, and Skye 

 the mean of several hundred observations has given me similar results. 



It appears therefore that though there is sometimes extraordinary local diversity in 

 the direction of the dykes in those districts where they present the gregarious type, the 

 general north-westerly trend can usually still be recognised. But when we turn to the long 

 massive solitary dykes, we soon perceive a remarkable change in their direction as we 

 follow them northward into Scotland. In the paper just referred to, I pointed out how 

 the general north-westerly trend becomes east and west in the Lothians, with a tendency 

 to veer a little to the south of west and north of east. This departure from the normal 

 direction is now seen to be part of a remarkable radial arrangement of the dykes. 



* Trans. Roy. Soc. Edin., xx. p. 650. t Trans. Geol. Soc, iii. p. 225. 



