All the features of these trap-dykes here stated may be observed in the central district of Scotland, among rocks of Carboniferous age. But he who would study trap-dykes on the great scale without quitting Britain, should visit some of the more trappean islands of the Hebrides. He will there find them intersecting glen and hill-side, in an intricate network, standing up through the heather like ruined walls, and running often for considerable distances up bald cliff-line, and across precipitous ravine. In some localities, among such limestone districts as that of Strath, detached eminences may be seen with congregated dykes coursing their sides and summits, while the heathy interspaces are cumbered with grey and white protruding blocks of marble, that give to these green knolls the aspect of old time-wasted abbeys with their clustering tombstones. The magnificent sections laid open in these localities by the action of mountain streams, and by the waves of the Atlantic, leave the student of igneous rocks nothing to desire save a long lease of leisure. Another form frequently assumed by the intrusive traps, is that of wide beds or sheets intercalated with greater or less regularity among stratified rocks (Fig. 34 _b_). They may be regarded as horizontal dykes, the igneous matter, in place of cutting across the strata, having forced a way for itself between them. Viewed in this light they will be found exactly to correspond with ordinary dykes; the rocks on which they rest, and those which lie above them being both altered like those on either side of a dyke or vein. A well-known example of this form of trap is that of Salisbury Crags, where a bed of greenstone twenty to eighty feet thick is intercalated among sandstones, shales, and coarse limestones, belonging to the Lower Carboniferous series. Its under surface presents a remarkably even line, broken at intervals, however, where the truncated ends of sandstone beds protrude up into the greenstone, or where the latter cuts into the sandstone below, occasionally enveloping detached fragments, and sending veins through them. Along the line of contact both rocks undergo a change. The greenstone becomes reddened, finer grained, and of a dull earthy aspect. The sandstones and shales are also red, and excessively hard, the former resembling a quartz rock, and the latter passing into a sort of flinty chert or chalcedony. The sandstones above the trap, where they can be examined, are also found to present the same hardened, baked appearance, the most intense metamorphism being observable in those parts which are completely surrounded by igneous matter. These points were noted many years ago during the famous controversy between the disciples of Hutton and Werner, the former viewing them as demonstrative evidence of the igneous origin of the trap-rock, the latter, on the other hand, professing to see nothing in the section of the Crags at all militating against the theory that the rocks had