Among the massive rocks abundant examples of such special types of weathering may be cited, from the acid and basic series, and from superficial lavas as well as from intrusive bosses and sills. Acid bosses, such as those of granite, granophyre and quartz-porphyry, tend to weather into blocks and finally into sand, and as this tendency is somewhat uniformly distributed through the rocks, they are apt to assume rounded, dome-shaped or conical forms which, at a distance, may seem to have smooth declivities, but on examination are generally found to be covered with a slowly-descending sheet of disintegrated blocks and debris (Fig. 346). When less prone to decay, and especially where traversed by a strongly-defined system of vertical joints, they may shoot up into tower-like heights, with prominent spires and obelisks. Basic bosses, when their materials decay somewhat rapidly, give rise to analogous topographical forms, though the more fertile soils which they produce generally lead to their being clothed with vegetation. Where they consist of an obdurate rock, much jointed and fissured, like the gabbro of the Inner Hebrides, they form exceedingly rugged mountains, terminating upward in serrated crests and groups of aiguilles (Figs. 331, 333). Acid lavas that have been superficially erupted weather into irregularly craggy hills, like the flanks of Snowdon. Those of intermediate composition, where they have accumulated in thick masses, are apt to weather into conical forms, as may be seen among the Cheviot, Pentland and Garleton Hills (Figs. 109, 110, 133); but where they have been poured out in successive thin sheets they have built up undulating plateaux with terraced sides, as among the Ayrshire and Campsie Fells and the hills of Lorne (Figs. 99, 107). Basic lavas have issued in comparatively thin sheets, frequently columnar or slaggy, forming flat-topped hills and terraced escarpments, such as are typically developed among the Tertiary basalt-plateaux of the Inner Hebrides and the Faroe Islands (Figs. 11, 265, 283, 284, 286).