CONCRETIONARY &C. STRUCTURES OF ROCKS. 167 of different sizes, but of various irregular forms. It is not unfrequent for these laminae to be curved, so as to have a convexity and a concavity ; while, in other cases, all their boundaries are convex, causing the laminar to approximate at length to a large spheroidal structure. Further, they pass into the cuboidal or square prismatic structure, in consequence of fissures at right angles to their planes; and, in the same manner, they are sometimes split into imperfect co lumnar divisions. The minuteness of the laminar structure is at times such, that granite possessing this character has been called schistose ; but the difficulty which attends some cases of this nature is examined in the chapter on the Destruction of Rocks. (Chap, xiii.) It is proper here to add, that the larger laminar structure is most frequent in granite ; but that it occurs in some of the trap rocks, including porphyries, and is, in particular, very conspicuous in hypersthene rock. The smaller laminae are found principally in the traps and in pitchstones : and it thus appears that this structure is nearly peculiar to the unstratified rocks. It occasion ally happens that the laminar structure is to be discovered only after exposure to the air, a circum stance necessarily noticed in the chapter on decompo sition, and that it may be combined with other varie ties, as with the columnar, in many of the trap family. It is also found in the veins of the later traps and the antient porphyries, as well as in the products of volcanoes. The circumstances thus detailed respecting the rocks to which this structure belongs, added to a careful and unprejudiced eye, must be the Geologist’s guide in distinguishing laminae from strata, a concre tionary form from a real stratification.