A study f the characters of the Tertiary dyke-swarms of Skye, Ardnmurchan and. the Small Isles of Invernessshire is based on observations at over 7500 dyke-outcrops, most of whiciL are located along well-exposed traverses. Because of geological limitations analyses of the properties of the dykes (trend, dip and thickness) can be semi-statistical only. Each of the dyke-swarms consists of a regional linear component of LW. to N.N.W. trending dykes, including parallel secondary-swarms in Skye and Ardnamurchan, together with in Skye and R.hum N.E. subswarnis of distinctive geo-. graphic distribution and comparatively- low intensities. The observations taken have facilitated: (a.) the construction of contour-maps depicting symmetrical distributions of multiple- dykes, of the arithmetic-average trends and thicknesses of the dykes, etc., (b.) both an approximate delimitation of regional-swarms and the distinction of these from subsidiary-avarms, in each case on the basis of the intensity- distribution and trend-distribution of the constituent dykes, and Cc.) the discovery of a latent plutonic -complex near Jluck, az a possible "centre" off the rwrth-west coast of Lewis. The trends, thicknesses, and to some extent the dips of small groups of dykes are intimately related to: (i.) the locations of those dykes with respect to the axes of highintensity of both crustal-stretch (dilation) and number of dykes per kilometre, (ii.) the positions of the dykes in relation to the site of the roughly contemporaneous Central Intrusive Coiiplexes, and (iii.) the structure of the country-- rock in which the dykes were emplaced, especially in the cases of the !oinian rocks, the Tertiary lavas, and the peripherally folded Mesozoic rocks bordering the Central Complex of Skye. The form and structure of the dyke-swarms, the distribution of the petrological types of dykes, and the available aeromagnetic, gravity-anomaly, and radiometric-dating evidence, indicate that the emplacement of each dyke-swarm is related to a sone of N.W. transcurrent faulting roughly paralleling the major dilation-axis of the swarm. Such faults were the consequence of differential movement of crustal blocks away from the line of a proposed Tertiary separation (Rockall Trough) of the British mainland and Rockall Plateau. Intracrustal, elongate, ridge-like basaltic magma-reservirs, whose ultimate source was the upper mantle, are believed to have ascended each of these faults. At the intersections of the N.W. transcurrent faults with preexisting N.E. faults cylinders of basaltic magma arose to form the Central Intrusive Complexes. Dyke-swarms developed as offshoots of the basaltic ridges and to a small extent from the basaltic cylinders, under the influence of a N.E. to S.W. tension resulting from a relative separation in this same direction of' the crustal blocks on both sides of the Rockall Trough rift.