spherules, generally hollow, resembling the roe of a fish. This structure is confined to limestones. Crystalline. When formed of distinct crystals of the same, or different, minerals united into a solid mass. This structure is sometimes invisible to the naked eye in the case of many crystalline rocks, but is rendered apparent under the microscope. In granites, porphyries, and some marbles, the crystalline struc ture is easily discerned by the naked eye. Saccharoid. When the structure and appearance resembles that of crystallized sugar; such as statuary marble. Fibrous. When the stone appears as if composed of a number of fibres ; such as a species of gypsum. Cellular or vesicular. When containing numerous cavities; such as some kinds of lavas. Schistose. When, owing to the internal structure and arrangement of the particles, the rock is capable of being split into laminae, or slates, as in the case of clay-slate, which is due to ‘ slaty cleavage.’ To this subject I have already referred (p. 287). Concretionary. When the rock, is formed of, or contains, concentric layers arranged generally round a nucleus. Large concretions, generally iron-stained, are frequently found amongst sandstone rocks, and are a source of much inconvenience and loss to the workmen. Such concretions are useless for building purposes, and are generally rejected by quarrymen. x 2
