Chapter vi

. (vol. i. p. 81), we have to look for the finer-grained marginal strip at the edge of a dyke, which, where traceable across another dyke, marks at once their relative age. The cross joints of the two dykes also run in different directions. Reference may again be made to the illustration given in Fig. 253 where three distinct groups of dykes intersect each other as they traverse the Lias limestones of Skye. The chilled edges and the different arrangement of joints mark these dykes out from each other, while the order in which they cross each other furnishes a clue to their relative age. If from such sections, repeated in different parts of a district, certain persistent petrographical characters can be ascertained to distinguish each particular system of dykes, a guide may thereby be obtained for the chronological grouping of the intrusions even where evidence of actual intersection is not visible. In the case just cited from Skye, the later north and south dykes are characterized by their lines of vesicular cavities and by the large porphyritic felspars which they contain.

[Footnote 205: _Trans. Geol. Soc._ iii. p. 75.]

It is obvious, however, that although sections of this kind suffice to prove the dykes to belong to distinct periods of intrusion, no longer interval need have elapsed between their successive production than was required for the solidification and assumption of a joint-structure by an older dyke before a younger broke through it. They may both belong to one brief period of volcanic activity. But when we pass to a series of dykes traversing a considerable district of country, and find that those which run in one direction are invariably cut by those which run in another, the inference can hardly be resisted that they do not belong to the same period of eruption, but mark successive epochs of volcanic energy. An excellent example of this kind of evidence is furnished by Mr. Clough from Eastern Argyleshire. The east and west dykes in that district are undoubtedly older than those which run in a N.N.W. direction (Fig. 257).[206] The latter are by far the most abundant, and are on the whole much narrower, less persistent, and finer in grain. On the opposite coast of the Clyde, a similar double set of dykes may be traced through Renfrewshire, those in an east and west direction being comparatively few, while the younger N.N.W. series is well developed. The great sheets or "sills" connected with one of the Stirlingshire dykes, already described, appear to me to furnish similar evidence in the younger dykes which run through them. And this evidence is peculiarly valuable, for it shows a succession even among adjacent dykes which all run in the same general direction.

[Footnote 206: As already stated, Mr. Clough and also Mr. Gunn are inclined to separate these older east and west dykes from the Tertiary series and to regard them as probably of late Palæozoic age.]

But in all these cases it is obvious that we have little indication of the length of time that intervened between the successive injections of the dykes. In Skye, however, more definite evidence presents itself that the interval must have been in some cases a protracted one. As far back as the year 1857,[207] I showed that the basic dykes of Strath in Skye are of two ages; that one set was erupted before the appearance of the "syenite" (granophyre) of that district, and was cut off by the latter rock; and that the other arose after the "syenite" which it intersected. Recent re-examination has enabled me to confirm and extend this observation. The younger series which traverses the granophyre is much less numerous than the older series in the same districts. In