[185] Trans. Roy. Soc. Edin. xxii. (1861), p. 650.
[186] Trans. Geol. Soc. iii. p. 225.
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 recognized. 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. I formerly 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.[187] This departure from the normal direction is now seen to be part of a remarkable radial arrangement of the dykes. 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 proceeding 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 east and west, 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 Map I.).
[187] Trans. Roy. Soc. Edin. xxii. p. 651.
Fig. 241.—Section along the line of the Cleveland Dyke at Cliff Ridge, Guisbrough (G. Barrow).
Scale, 12 inches to 1 mile.
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, is said to end off abruptly against the floor of the limestone.[188] 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 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.
[188] This is expressed in the Geological Survey Map, Sheet 93, N.E.
Excellent illustrations of this feature are supplied by sections on the line of the Cleveland dyke. Towards its south-easterly extremity, this great band of igneous rock ascends from the low Triassic plain of the Tees into the high uplands of Cleveland. Its course across the ridges and valleys there has been carefully traced for the Geological Survey by Mr. G. Barrow, who has shown that over certain parts of its course it does not reach the surface, but remains concealed under the Jurassic rocks, which it never succeeded in penetrating. But that in places it comes within a few feet of the soil is shown by the baked shale at the surface, for the alteration which it has induced on the surrounding rocks only extends a few feet from its margin. These interruptions of continuity show how uneven is the upper limit of the dyke. The characteristic porphyritic rock may be observed running up one side of a hill to the crest, but never reaching the surface on the other side. At Cliff Ridge, for example, about three miles south-west of Guisbrough, Mr. Barrow has followed it up to the summit on the west side; but has found that on the east side it does not pierce the shales, which there form the declivity. This structure is represented in Fig 241. The vertical distance between the summit to the left, where the dyke (b) disappears, and the point to the right, where the Lias shale (a) of the hillside is concealed by drift (c), amounts to 250 feet, the horizontal distance being a little more than 900 feet. But as the shale when last seen at the foot of the slope is quite unaltered, the dyke must there be still some little distance beneath the surface, so that the vertical extension of this upward tongue of the dyke must be more than 250 feet. Mr. Barrow, to whom I am indebted for these particulars, has also drawn the accompanying section ([Fig. 242]) along the course of the dyke for a distance of nearly 11 miles eastward from the locality represented in [Fig. 241]. From this section it will be observed that in that space there are at least three tongues or upward projections of the upper limit of the dyke. Several additional examples of the same structure are to be seen further east towards the last visible outcrop of the dyke.