But besides the horizontally-disposed intrusive sheets and laccolites, great, radiating, vertical fissures are produced by the heaving forces acting beneath those volcanic centres which have been closed up and 'cicatrised' by the exudation from them of subterranean materials. These vertical intrusions, which we call dykes, like the horizontal ones, differ in character, according to the nature of the materials of which they are composed. Dykes of acid and intermediate lava are usually of considerable width, and do not extend to great distances from the centres of eruption. Dykes composed of the more-liquid, basic lavas, on the other hand, may extend to the distance of hundreds of miles from the central vent. The way in which comparatively narrow, basaltic dykes are found running in approximately straight lines for such enormous distances is a very striking fact, and bears the strongest evidence to the heaving and expanding forces at work at volcanic centres, during and subsequently to the extrusion of the igneous products at the surface. These basaltic dykes occur in such prodigious numbers around some volcanic vents, that the whole of the stratified rocks in the immediate vicinity are broken up by a complete network of them, crossing and interlacing in the most complicated fashion. Farther away from the vents, similar dykes are found in smaller numbers, evidently radiating from the same centre, and sometimes extending to a distance of more than a hundred miles from it. Nowhere can we find more beautiful illustrations of such dykes than in the Western Isles of Scotland. When composed of materials which do not so easily undergo decomposition as the surrounding rocks, they stand up like vast walls; but when, on the other hand, they are more readily acted on by atmospheric moisture than are the rocks which enclose them, they give rise to deep trenches with vertical sides, which render the country almost impassable. [Sidenote: STRUCTURE OF INTRUSIVE MASSES.] The lava consolidating in these horizontal intrusions (sheets and laccolites), and the vertical intrusions (dykes), is usually more crystalline in structure than the similar materials poured out at the surface. In the same dyke or sheet, when it is of great width, we often find every variation--from a glassy material formed by the rapid cooling of the mass where it is in contact with other rocks, to the perfectly crystalline or granitic varieties which form the centre of the intrusion. It is in these dykes and other intrusions that we find the most convincing evidence of the truth of the conclusions, which we have enunciated in a former chapter, concerning the dependence of the structure of an igneous rock upon the conditions under which it has consolidated. One material is found, under varying conditions, assuming the characters of obsidian, rhyolite, quartz-felsite, or granite; another, under the same set of conditions, taking the form of tachylyte, basalt, dolerite, and gabbro.