Fig. 237.—Systems of joints in the dykes.
a, parallel; b, transverse.
Two distinct systems of joints are recognizable (Fig 237). Though sometimes combined in the same dyke, they are most conspicuously displayed when each occurs, as it generally does, by itself. The first and less frequent system of joints (a) has been determined by lines of retreat, which are parallel to the walls of the dyke. The joints are then closest together at the margin, and may be few or altogether absent in the centre. They are sometimes so numerous, parallel and defined towards the borders of the dyke, as to split the rock up into thin flags. Where transverse joints are also present these flags are divided into irregular tesseræ.
Fig. 238.—Section of cylindrical vein or dyke, cutting the bedded lavas, east side of Fuglö, Faroe Islands.
In the second or transverse system of joints (b), which is the more usual, the divisional lines pass across the breadth of the dyke, either completely from side to side, or from one wall for a longer or shorter distance towards the other. Where this series of joints is most completely developed the dyke appears to be built up of prisms piled horizontally, or nearly so, one above another. These prisms, in rare instances, are as regular as the columns of a basalt-sheet (see [Fig. 166]). Usually, however, they have irregularly defined faces, and merge into each other. Where the prismatic structure is not displayed, the joints, starting sharply at the wall of the dyke, strike inwards in irregular curving lines. It is such transverse joints that enable the eye, even from a distance, to distinguish readily the course of a dyke up the face of a cliff of basalt-beds, for they belong to the dyke itself, are often at right angles to those of the adjacent basalt, and by their alternate projecting and re-entering angles seam the dyke with parallel bars of light and shade (see the double dyke in [Fig. 333]). Where they traverse not only the general mass of a dyke, but also the "contemporaneous veins" which cross it, it may be inferred that these veins were injected before the final solidification and contraction of the whole dyke.
An interesting modification of the transverse joints may sometimes be observed, where, as in the case of the Palæozoic "Rock and Spindle," at St. Andrews ([Fig. 222]), the molten material has solidified in a tubular or spherical cavity. The joints then radiate inwards from the outer curved surface. The most remarkable instance of this structure which I have found among the Tertiary volcanic plateaux occurs on the east side of the island Fuglö, the most north-easterly of the group of the Faroes. It is cut in section by the face of the precipice, where it appears as a round mass about 40 or 50 feet in diameter piercing the plateau-basalts. A selvage of finer material round its outer edge shows the effect of rapid chilling, while the joints diverge from the periphery and extend in fan-shape towards the centre ([Fig. 238]).
Fig. 239.—Joint-structures in the central vitreous portion of the Eskdale Dyke (B. N. Peach).
A, View of a square yard of the outer wall of the vitreous central band, showing the polygonal arrangement of the prisms and their investing sheath of ribs.
B, View of a smaller portion of the same wall to show the detailed structure of the ribs (a a) and their vitreous cores (b b).
C, Profile of a part of the weathered face of the wall, showing the way in which the hard ribs or sheaths project at the surface.
One of the most remarkable exhibitions of joint-structure hitherto noticed among the Tertiary dykes is that which occurs in the central vitreous band of the Eskdale dyke already referred to. The rock is divided into nearly horizontal prisms, each of which consists of an inner more vitreous core and an outer more lithoid sheath. By the coherence of their polygonal and irregular faces, and the greater durability of their material, these sheaths project on the weathered wall of the vitreous centre of the dyke in a curiously reticulated grouping of prominent ribs each about two inches broad ([Fig. 239], A), while the vitreous cores, being more readily acted on by the weather, are hollowed out into little cup-shaped depressions. Each rib is thus composed of the sheaths or outer lithoid portions of two prisms, the line of separation being marked by a suture along the centre (B). Between this median suture and the inner glassy core the rib is further cut into small segments by a set of close joints, which are placed generally at right angles to the course of the rib (C). Examined with a lens, the lithoid substance of these sheaths has a dull finely granular aspect, like that of felsitic rocks, with scattered felspars. It is obviously a more devitrified condition of the material which forms the core of each prism. This material presents on a fresh fracture a deep iron-black colour, dull resinous lustre and vitreous texture. It at once recalls the aspect of many acid pitchstones, and in the early days of petrography was naturally mistaken for one of these rocks. Through its substance numerous kernels of more glassy lustre are dispersed, each of which usually contains one or more amygdales of dull white chalcedony, but sometimes only an empty black cavity. These black glistening kernels of glass, of all sizes up to that of a small bean, scattered through the dull resinous matrix, form with the white amygdales the most prominent feature in the cores; but crystals of felspars may also be observed. Some details of the microscopic characters of this remarkable structure will be given in a subsequent page. The relation of the cores and sheaths to the prismatic jointing of the rock seems to show that devitrification had not been completed when these joints were established, and that it proceeded from the faces of each prism inwards.