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FRACTURE.—INDETERMINATE FORMS. 57 The lamina 1 may be either flat or curved. When the lamina) are so firmly held together as to allow of division only into plates of variable thickness, the structure is termed slaty. These laminar and slaty structures also produce more or less perfect cleavage planes. Jfibrous, the fibres varying in size from those of the most e icate asbestus to large and rigid, in which condition the structure is said to be Columnar ; but as it passes by insensible degrees from fine to coarse, there is no apparent limit to either character, nor has ail mi . ass ig n ed by which they may be distinguished. t he fibrous may lie either straight or bent, and their relative ^° 8 ' 0118 ma y be parallel, divergent, radiating, promiscuous, or ma ted, which structure is generally the cause of toughness. granular, either large or small. When the grains cohere slightly, the mineral is termed sandy, earthy, pulverulent, as chalk; where the cohesion is grea er, the mineral is said to be compact. ie grains are sometimes small regularly-formed crystals, or oyV/K'I. ° masses > which, on breaking the specimen, may mi se parately their own proper structure. ,e structure is frequently alike in different minerals; and cnee it ceases to he an exact determinative character. FRACTURE. 236. This term is used to denote the uneven and irregular sur- aces produced by breaking minerals. These are not, however, to c relied upon as distinctive characters ; for different specimens the same mineral will exhibit different forms of fracture, according to their more or less pure and perfect state, and, probably, according to the direction in which the fracture takes place. 1 he following are the kinds of fracture usually described:— Conchoidal, as in flint. Uneven. Splintery, when the broken surface exhibits protruding points. -//ncHy, as in some metals when broken. the uneven and splintery fractures are common to so many minerals as to render those characters of comparatively little value in determining species. INDETERMINATE FORMS. 237. The following are some of the irregular and accidental shapes of minerals, the varieties of which are too numerous to