48 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



for it may be observed over ground where the naked rocks come almost everywhere to 

 the surface, and where, therefore, if the conspicuous material of the dykes existed, it 

 could not fail to be found. No dyke supplies better illustrations of this discontinuity 

 than that of Cleveland. Traced north-westward across the Carboniferous tracts that 

 lie between the mouth of the Tees and the Vale of the Eden, this dyke disappears some- 

 times for a distance of six or eight miles. In the mining ground round the head of 

 the South Tyne the rocks are bare, so that the absence of the dyke among them 

 can only be accounted for by its not reaching the surface. Yet there can be no doubt 

 that the various separated exposures, which have the same distinctive lithological characters 

 and occur on the same persistent line, are all portions of one dyke which is continuous 

 at some depth below ground. We have thus an indication of the exceedingly irregular 

 upward limit of the dykes, as will be more particularly discussed further on. 



But there are also instances where the continuity is interrupted and then resumed 

 on a different line. One of the best illustrations of this character is supplied by the 

 large dyke which rises through the hills about a mile south of Linlithgow and runs 

 westward across the coal-field. At Blackbraes it ends off in a point, and is not found 

 again to the westward in any of the coal-workings. But little more than quarter of a 

 mile to the south a precisely similar dyke begins, and strikes westward parallel to the 

 line of the first one. The two separated strips of igneous rock overlap each other for 

 about three-quarters of a mile. But that they are merely interrupted portions of what 

 is really a single dyke can hardly be questioned. A second example is furnished by 

 another of the great dykes of the same district, which after running for about 12 miles 

 in a nearly east and west direction suddenly stops at Chryston, and begins again in the 

 same direction, but on a line about a third of a mile further north. Such examples 

 serve to mark out irregularities in the great fissures up which the materials of the 

 dykes rose. 



§ 7. Length. 



In those districts where the small and crowded dykes of the gregarious type 

 are developed, one cannot usually trace them for more than a short distance. The 

 longest examples known to me are those which have recently been mapped with much 

 patience and skill by Mr Clough in Eastern Argyleshire. Some of them he has been 

 able to track over hill and valley for four or five miles, though the great majority are 

 much shorter. In Arran and in the Inner Hebrides, it is seldom possible to follow what 

 we can be sure is the same dyke for more than a few hundred yards. This difficulty 

 arises partly, no doubt, from the frequent spread of peat or other superficial accumulation 

 which conceals the rocks, and partly also from the great number of dykes and the want 

 of sufficiently distinct lithological characters for the identification of any particular one. 

 But making every allowance for these obstacles, we are compelled, I think, to regard the 

 gregarious dykes as essentially short as well as relatively irregular. 



In striking contrast to these, come the great solitary dykes. In estimating their 



