The question how far an ascending mass of magma can melt down its walls is one to which no definite answer can yet be given. Recent observations show that where the difference in the silica percentage between the magma and the rock attacked is great, there may be considerable dissolution of material from this cause. Allusion has already been made to Mr. Harker's computation that some of the acid granophyres of Skye have melted down about a fourth of their bulk of the basic gabbros. If such a reaction should take place between the magma of a boss, sill or laccolite and the rocks among which it has been intruded, great changes might result in the composition of the intruded rock. We are not yet, however, in possession of evidence to indicate that absorption of this kind really takes place on an extensive scale within the earth's crust. If it did occur to a large extent, we should expect much greater varieties in the composition of eruptive rocks than usually occur, and also some observable relation between the composition of the igneous material and that of the rocks into which it has been injected. But enough is not yet known of this subject to warrant any decided opinion regarding it. CHAPTER VII Influence of Volcanic Rocks on the Scenery of the Land--Effects of Denudation. As considerable popular misapprehension exists respecting the part which volcanism has played in the evolution of the existing topography of the earth's surface, and as the British Isles, from their varied geological structure, offer special facilities for the discussion of this subject, it may not be out of place to devote a final section of the present Introduction to a consideration of the real topographical influence of volcanic action. With modern, and especially with active, volcanoes we need not here concern ourselves. Their topographical forms are well known, and give rise to no difficulty. The lofty cones of the Vesuvian type, with their widespread lavas and ashes, their vast craters and their abundant parasitic volcanoes; the crowded, but generally diminutive, cones and domes of the puy type, so well displayed in Auvergne, the Eifel and the Bay of Naples; and the vast lava deserts of the plateaux, so characteristically developed in Iceland and Western America, illustrate the various ways in which volcanic energy directly changes the contours of a terrestrial surface. But the circumstances are altered when we deal with the topographical influence of long extinct volcanoes. Other agencies then come into play, and some caution may be needed in the effort to disentangle the elements of the complicated problem, and to assign to each contributing cause its own proper effect.