In tracing the great solitary dykes across the country, the geologist is often surprised to meet with gaps, varying in extent from a few hundred feet to several miles, in which no trace whatever of the igneous rock can be detected at the surface. This disappearance is not always explicable by the depth of the cover of superficial accumulations; 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 Yale of the Eden, this dyke disappears sometimes 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 a 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 twelve 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