In the West of Scotland, where dykes are so abundantly developed, considerable differences can be observed between the amount of metamorphism superinduced by adjacent dykes which may be of the same thickness, and cut through the same kind of strata. Such variations have not probably arisen from differences in the temperature of the original molten rock. Perhaps they are rather to be assigned to the length of time occupied by the ascent of the lava in the fissure. If, for instance, the fissure opened to the surface and discharged lava there, the rocks of its walls would be exposed to a continuous stream of molten rock as long as the outflow lasted. They would thus have their temperature more highly raised, and maintained at such an elevation for a longer time than where the magma, at once arrested within the fissure, immediately proceeded to cool and consolidate there. It would be an interesting and important conclusion if we could, from the nature or amount of their contact-metamorphism, distinguish those dykes which for some time served as channels for the discharge of lava above ground. Some dykes which have caught up fragments of older rocks in their ascent have exercised a considerable solvent action on these inclusions. Examples of this feature have already been cited from Skye, where they have been studied by Mr. Harker (pp. 129, 163). In connection with the metamorphism superinduced by dykes, reference may again be made to the alteration which they themselves undergo where they have invaded a carbonaceous shale or coal. The igneous rock, as we have seen, loses its dark colour and obviously crystalline structure, and becomes a pale yellow or white, dull, earthy substance, or "white trap." The chemical changes involved in this alteration have been described by Sir J. Lowthian Bell.[202] Dr. Stecher has also discussed the alterations traceable by the aid of the microscope.[203] Though most of the instances of such transformation in Britain occur in the Carboniferous system, and have taken place in intrusive rocks of probably, for the most part, Carboniferous or Permian age, yet they are not unknown in the Tertiary volcanic series. Some of the "white trap" of the Coal-measures may indeed belong to the Tertiary period, but the coals and carbonaceous shales interstratified in the Tertiary basalt-plateaux have reacted on both the superficial lavas and the sills, and have given rise to the same kind of alteration as in the Carboniferous system, as will be shown in a later Chapter. [Footnote 202: _Proc. Roy. Soc._ xxiii. (1875), p. 543.] [Footnote 203: Tschermak's _Mineralogische Mittheilungen_, ix. (1887), p. 145, and _Proc. Roy. Soc. Edin._ 1888.] [Illustration: Fig. 256.--Dolerite dyke with marginal bands of "white trap," in black shale, Lower Lias, Pabba.