The dykes of Scotland

Abstract

I. Introduction. It is fitting that in Scotland, where the term dyke was first applied,1 there should occur more numerous and more varied dyke-assemblages than are known from any other area of equal size. During seven igneous periods, in which magmas of many different types were available for injection, the country north of the Tweed was subjected to dyke-fracturing, and myriads of these wall-like intrusions were formed. Sir James Hall was the first to realise during a visit to Vesuvius towards the end of the eighteenth century that dyke-fissures were filled by the uprise of magma from below, and not as the Wernerians believed from above, by some mysterious infiltrations from the sea. To us in Edinburgh, direct descendants of the school which Hutton, Playfair, and Hall founded, it is of especial interest that their controversy with the Wernerians was clinched to their satisfaction within the city’s boundaries by the dyke which crosses Salisbury Crags and which served as a final argument to prove beyond the possibility of a doubt that basalt was once molten. How relatively little more we know even now! For each intrusion we picture a fissure through which molten rock once flowed and in which it congealed, but almost always the source from which the magma came is unseen and the ultimate destination uncertain. Still, in Scotland, abundance of material has led to a full measure of scientific achievement, and the prolonged investigation carried on by successive generations of geologists has brought in much knowledge. With