The Sill is traversed by the same faults which disrupt the surrounding Carboniferous rocks. It is therefore of older date than these dislocations. Its striking general parallelism with the shales and limestones probably proves that it was intruded before the rocks were much disturbed from their original horizontal position. But the manner in which the intrusive rock has been thrust into and has involved the shales and limestones seems to indicate that these strata had already become consolidated and lay under the pressure of a great thickness of superincumbent Carboniferous strata. In the absence of all certainty on the subject it seems most natural to place the Whin Sill provisionally among the Carboniferous volcanic series with which petrographically and structurally it has so much in common. In Scotland the puy-eruptions continued till the time of the Coal-measures. If, before the close of the Carboniferous period, volcanic vents were opened somewhere to the east of the coal-fields of Northumberland and Durham, they might be accompanied with basic sills injected into the Carboniferous Limestone series, which was then lying still approximately horizontal under a thickness of from 3500 to 5000 feet of Carboniferous sedimentary deposits. These still undiscovered volcanoes seem to have been endowed with even more energy than those of Central and Southern Scotland, at least nowhere else among the Carboniferous records of Britain is there such a colossal manifestation of subterranean intrusion as the Great Whin Sill. 2. THE DERBYSHIRE TOADSTONES In the absence of any certain evidence that the Whin Sill belongs to the Carboniferous period, we must advance southward into the very heart of England before any clear vestiges can be found of contemporaneous volcanic eruptions among the members of the Carboniferous system. After quitting the lavas and tuffs of Roxburghshire and their brief continuations across the English border, we do not again meet with any truly bedded volcanic rocks in that system until we reach the middle of Derbyshire. In this picturesque district, famous for its lead-mines and its mineral waters, a feebly developed but interesting group of intercalated lavas, locally called "toadstones," has long been known. There is thus a space of some 150 miles across which, though the formations are there so fully developed and so abundantly trenched by valleys from the top to the bottom of the system, no volcanic vents nor any trace of Carboniferous volcanic ejections has yet been found. On the other hand, after the district of the "toadstones" is passed, the Carboniferous rocks are again destitute of any volcanic intercalations across the centre and south-west of England and over Wales, until after a space of about 150 miles they reappear in Somerset.