1st. Thick, massive, prismatic or rudely-jointed sheets, rather more coarsely crystalline and obviously more durable than the other types, inasmuch as they project in tabular ledges and tend to retain perpendicular faces owing to the falling away of slices of the rock along lines of vertical joints. Many rocks of this type are undoubtedly intrusive sheets, and as such will be further referred to in a later chapter. But the type includes also true superficial lavas which show the characteristic slaggy or vesicular bands at their upper and lower surfaces. The mere presence of such bands may not be enough, indeed, absolutely to establish that the rock possessing them flowed at the surface as a lava, for they are occasionally, though it must be confessed rarely, exhibited by true sills. But the rough scoriaceous top of a lava-stream, and the presence of fragments of this surface in the overlying tuff, or wrapped round by the next succeeding lava, sufficiently attest the true superficial outflow of the mass. 2nd. Prismatic or columnar basalts, which, as at the Giant's Causeway and Staffa, have long attracted notice as one of the most striking topographical elements of the plateaux. Columnar structures are typical of the more compact heavy basalts. A considerable variety is observable in the degree of perfection of their development. Where they are least definite, the rock is traversed by vertical joints, somewhat more regular and close-set than those in the dolerites, by the intersection of which it is separated into rude quadrangular or polygonal columns. The true columnar structure is shown in two chief forms. (_a_) The rock is divided into close-fitting parallel, usually six-sided columns; the number of sides varying, however, from three up to nine. The columns run the whole thickness of the bed, and vary from 8 or 10 to 40 or even 80 feet in length. They are segmented by cross joints which sometimes, as at Giant's Causeway, take the ball-and-socket form. Occasionally they are curved, as at the well-known Clam-shell cave of Staffa. (_b_) The prisms are much smaller, and diverge in wavy groups crowded confusedly over each other, but with a general tendency upwards. This starch-like aggregation may be observed superposed directly upon the more regular columnar form as at the Giant's Causeway and also at Staffa. Excellent illustrations of both these types may be seen at many points along the sea-cliffs of the Inner Hebrides; the western coast of Skye, the south-west side of Mull, and the cliffs of the island of Canna may be specially cited. [Illustration: Fig. 259.--Section of scoriaceous and prismatic Basalt, Camas Tharbernish, north shore of Canna Island.]