52 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Beginning at the southern margin of the dyke-region, we have the notable example of 

 the Cleveland dyke, which in its course from Cleveland to Carlisle runs nearly W. 15° N. 

 The Eskdale dyke has an average trend of W. 32° N., and the same general direction is 

 maintained by the group of dykes which run from the Southern Uplands across the south- 

 west of Lanarkshire and north-east of Ayrshire. But as we proceed northwards we 

 observe the trend to turn gradually round towards the west. The dyke that runs from 

 near the mouth of the Coquet across the Cheviot Hills to beyond Hawick has a general 

 course of W. 8° N. In the great central coal-field of Scotland the average direction may 

 be taken to be nearly E. and W., the same dyke running sometimes to the north, and 

 sometimes to the south of that line. But immediately to the north a decided tendency 

 to veer round southwards makes its appearance. Thus the long dyke which runs from the 

 Carse of Stirling through the Campsie Fells to the Clyde west of Leven, has a mean direction 

 of W. 5° S. This continues to be the prevalent trend of the remarkable series of 

 dykes which crosses the Old Red Sandstone plains, though some of these revert in whole 

 or in part to the more usual direction by keeping a little to the north of west. Even as 

 far as Loch Tay and the head of Strathardle, the course of the dykes continues to be to 

 the south of west. Tracing these lines upon a map of the country we perceive that they 

 radiate from an area lying along the eastern part of Argyleshire and the head of the 

 Firth of Clyde (see Plate I.). 



§ 10. Termination Upwards. 



It was pointed out many years ago by Winch that some of the dykes which traverse 

 the Northumberland coal-field do not cut the overlying Magnesian Limestone. The 

 Hett dyke, south of Durham, which no doubt belongs to the ancient series of igneous 

 protrusions already referred to, is said to end off abruptly against the floor of the 

 limestone.* Here and there, among the precipices of the Inner Hebrides, a dyke may 

 be seen to die out before it reaches the top of the cliff. But in the vast majority of 



Fig. 6. —Section along the line of the Cleveland Dyke at Cliff Ridge, Guisbrough (G. Barrow), scale, 12 inches to 1 mile. 



cases, no evidence remains as to how the dykes terminated upwards. I have referred to 

 the occasional interruptions of the continuity of a dyke, where, though the rock does not 

 reach the surface, it must be present in the fissure underneath. Such interruptions 

 show that, in some places at least, there was no rise of the rock even up to the level of 

 what is now the surface of the ground, and that the upward limit of the dykes must 

 have been exceedingly irregular. 



* This is expressed in the Geological Survey map, Sheet 93, N.E. 



