(_a_) Where the banding is due to differences of crystalline texture, the constituent felspars, augites, and iron-ores may be seen even with the naked eye as well-defined minerals along the prominent surfaces of the harder ribs, while the broader intervening flutings of finer material show the same minerals in minuter forms. The alternating layers of coarser and finer crystallization lie, on the whole, parallel with the upper and under surfaces of the sheets in which they occur. But they likewise undulate like the streaky lines in ordinary flow-structure. Banded structure of this type may be seen well developed in the lower parts of the basalt-plateaux throughout the Inner Hebrides and the Faroe Islands. A specimen taken from the west end of the island of Sanday, near Canna, which showed the structure by a conspicuous parallel fluting on weathered surfaces, was sliced for microscopical examination. Mr. Harker has been kind enough to supply me with the following observations regarding this slice:-- "In the slice [6660][224] the banding becomes less conspicuous under the microscope. The rock is of basaltic composition, and, with reference to its micro-structure, might be styled a fine-grained olivine-diabase or olivine-dolerite in some parts of the slice, an olivine-basalt in others. It consists of abundant grains of olivine, imperfect octahedra and shapeless granules of magnetite, little simple or twinned prisms of labradorite, and a pale brown augite. The last-named mineral is always the latest product of consolidation, but it varies in habit, being sometimes in ophitic patches moulded upon or enclosing the other minerals, sometimes in small granules occupying the interstices between the felspars and other crystals. The ophitic habit predominates in the slice, while the granulitic comes in especially along certain bands. If the former be taken as indicative of tranquil conditions, the latter of a certain amount of movement in the rock during the latest stages of its consolidation, the banding, though not strictly a flow-structure, may be ascribed in some degree to a flowing movement of the nearly solidified rock. There is, however, more than this merely structural difference between the several bands. They differ to some extent in the relative proportions of the minerals, especially of olivine and augite; which points to a considerable flowing movement at an early stage in a magma which was initially not homogeneous." [Footnote 224: The figures within square brackets throughout the following pages refer to the numbers of the microscopic slides in the Geological Survey collection, where I have deposited all those prepared from my specimens.]