4th. Banded or stratiform lavas, consisting of successive parallel layers or bands which weather into projecting ribs and flutings. The deceptive resemblance to sedimentary rocks thus produced has no doubt frequently led to these lavas being mistaken for tuffs. As I have recently found them to be much more plentiful than I had supposed, a more detailed description of them seems to be required.
The banded character arises from marked distinctions in the texture of different layers of a lava-sheet. In some cases (a) these distinctions arise from differences in the size of the crystals or in the disposition of the component minerals of the rock; in others (b) from the varying number and size of the vesicles, which may be large or abundantly crowded together in some layers, and small or only sparsely developed in others. The structure thus points to original conditions of the lava at the time of its emission and may be regarded as, to some extent, a kind of flow-structure on a large scale.
(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."
[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.
(b) Where the banding arises from the distribution of the vesicles, somewhat similar weathered surfaces are produced. In some instances, while the basalt is throughout finely cellular, interposed bands of harder, rather finer-grained and less thoroughly vesicular character serve to give the stratified appearance. Instances may be observed where the vesicles have been crowded together in certain bands, which consequently weather out differently from the layers above and below them. An excellent illustration of this arrangement occurs in the lowest lava but one of the largest of the three picturesque stacks known as Macleod's Maidens on the west coast of Skye (Figs. 260, 283, 284 and 287). This lava is thoroughly amygdaloidal, but the vesicles are specially crowded together in certain parallel bands from an inch to three or four inches thick. Some of these layers lie close to each other, while elsewhere there may be a band of more close-grained, less vesicular material between them. But the most singular feature of the rock is to be seen in the shape and position of the vesicles that are crowded together in the cellular bands. Instead of being drawn out into flattened forms in the general direction of banding, they are placed together at high angles. Each layer remains parallel to the general bedding, but its vesicles are steeply inclined in one direction, which was doubtless that of the flow of the still unconsolidated lava.[225] Weathering along these bands, the lava might easily be mistaken at a little distance for a tuff or other stratified intercalation.
[225] This elongation of vesicles, more or less perpendicular to the general bedding, may be noticed sometimes even in sills, as will be shown in a later Chapter.
Fig. 260.—Banded amygdaloidal basalt showing layers of elongated and steeply inclined vesicles, Macleod's Maidens, Skye.