_a_, Black carbonaceous Lower Lias Shale; _b_ _b_, bands of indurated shale from 15 inches to 2 feet broad; _c_, dolerite dyke 3 feet 3 inches broad; _d_ _d_, bands of altered dolerite or "white trap," 3 to 5 inches broad. ] Some marked examples of this alteration of intrusive igneous material are to be observed among the basalt dykes which cut the Lower Lias Shales of Skye. These shales, where black and carbonaceous, as in the island of Pabba, have exercised an unmistakable influence on the abundant dykes which intersect them. The chilled selvage of each dyke has assumed the dull earthy pale-grey or yellowish aspect, which extends for a few inches from the wall into the interior, where it rapidly passes into the ordinary black crystalline basalt. These features will be readily understood from the accompanying diagram (Fig. 256). Where the dykes give off narrow veins a few inches broad, these consist entirely of the "white trap." The shales are often traversed with strong joints parallel to the walls of the dykes, and the transverse joints of the dykes are sometimes prolonged into the bands of indurated shale. 18. RELATION OF DYKES TO THE GEOLOGICAL STRUCTURE OF THE DISTRICTS WHICH THEY TRAVERSE. In no respect do the Tertiary dykes of Britain stand more distinguished from all the other rocks of the country than in their extraordinary independence of geological structure. The successive groups of Palæozoic and Mesozoic strata have been so tilted as to follow each other in approximately parallel bands, which run obliquely across the island from south-west to north-east. The most important lines of fault take the same general line. The contemporaneously included igneous rocks follow, of course, the trend of the stratified deposits among which they lie, and even the intrusive sills group themselves along the general strike of the whole country. But the Tertiary dykes have their own independent direction, to which they adhere amid the extremest diversities of geological arrangement.