Fig. 10.—Lumpy, irregular trachytic Lava-streams (Carboniferous), East Linton, Haddingtonshire.
4. The DISPOSITION OF LAVAS IN SHEETS OR BEDS is the result of successive outflows of molten rock. Such sheets may range from only a yard or two to several hundred feet in thickness. As a rule, though with many exceptions, the basic lavas, such as the basalts, appear in thinner beds than the acid forms. This difference is well brought out if we compare, for instance, the massive rhyolites or felsites of North Wales with the thin sheets of basalt in Antrim and the Inner Hebrides. The regularity of the bedded character is likewise more definite among the basic than among the acid rocks, and this contrast also is strikingly illustrated by the two series of rocks just referred to. The rhyolites and felsites, sometimes also the trachytes and andesites, assume lumpy, irregular forms, and some little care may be required to trace their upper and under surfaces, and to ascertain that they really do form continuous sheets, though varying much in thickness from place to place ([Fig. 10]). Like modern acid lavas, they seem to have flowed out in a pasty condition, and to have been heaped up round the vents in the form of domes, or with an irregular hummocky or mounded surface. The basalts, and dolerites, and sometimes the andesites, have issued in a more fluid condition, and have spread out in sheets of more uniform thickness, as may be instructively seen in the sea-cliffs of Antrim, Mull, Skye, and the Faroe Islands, where the horizontal or gently-inclined flows of basalt lie upon each other in even parallel beds traceable for considerable distances along the face of the precipices (Figs. [11], [265], and [286]). The andesites of the Old Red Sandstone (Figs. [99], [100]) and Carboniferous series (Figs. [107], [108], [111], [112], [113], [123]) in Scotland likewise form terraced hills.
The length of a lava-stream may vary within wide limits. Sometimes an outflow of lava has not reached the base of the cone from the side of which it issued, like the obsidian stream on the flanks of the little cone of the island of Volcano. In other cases, the molten rock has flowed for forty or fifty miles, like the copious Icelandic lava-floods of 1783. In the basalt-plateaux of the Inner Hebrides a single sheet may sometimes be traced for several miles.
Fig. 11.—View at the entrance of the Svinofjord, Faroe Islands, illustrating the terraced forms assumed by basic lavas. The island on the left is Borö, that in the centre Viderö, and that on the right Svinö.
Some lavas, more especially among the basic series, assume in cooling a Columnar structure, of which two types may be noticed. In one of these the columns pass with regularity and parallelism from the top to the bottom of a bed (Figs. [171], [225]). The basalt in which Fingal's Cave, in the isle of Staffa, has been hollowed out may be taken as a characteristic example (Fig. 266a). Not infrequently the columns are curved, as at the well-known Clam-shell Cave of Staffa. In the other type, the columns or prisms are not persistent, but die out into each other and have a wavy, irregular shape, somewhat like prisms of starch. These two types may occur in successive sheets of basalt, or may even pass into each other. At Staffa the regularly columnar bed is immediately overlain with one of the starch-like character. The columnar structure in either case is a contraction phenomenon, produced during the cooling and shrinking of the lava. But it is difficult to say what special conditions in the lava were required for its production, or why it should sometimes have assumed the regular, at others the irregular form. It may be found not only in superficial lavas but in equal perfection in some dykes and intrusive sills or injections, as among the Tertiary volcanic rocks of the island of Canna (Figs. [307] and [308]).
The precipitation of a lava-stream into a lake or the sea may cause the outer crust of the rock to break up with violence, so that the still molten material inside may rush into the water. Some basic lavas on flowing into water or into a watery silt have assumed a remarkable spheroidal sack-like or pillow-like structure, the spheroids being sometimes pressed into shapes like piles of sacks. A good instance of this structure occurs in a basalt at Acicastello in Sicily.[1] A similar appearance will be described in a later chapter as peculiarly characteristic of certain Lower Silurian lavas associated with radiolarian cherts in Britain and in other countries ([Fig. 12]).
[1] See Prof. G. Platania in Dr. Johnston-Lavis' South Italian Volcanoes, Naples (1891), p. 41 and plate xii.
