[221] Quart. Journ. Geol. Soc. vol. xlvi. (1890), p. 356. Professor Judd has there described under the name of "propylites" various members of the volcanic series which he believes to have undergone alteration from solfataric action. I have not been able to discover any trace of such action, but I have found that the lavas of the plateaux assume a peculiar condition where they have been affected by large intrusive masses of granophyre or gabbro. (See postea, [Chapter xlvi].)
(c) Rhyolites.—In the Antrim plateau a group of rhyolite bosses occurs, some of which have been claimed as superficial lavas. In some cases it can be demonstrated that they are intrusive, and in no instance can they be decisively shown to have escaped in streams at the surface. It is probable, however, that some of these bosses did actually communicate with the outer air, for between the lower and upper group of basalts in this plateau, bands of rhyolitic conglomerate occur which may indicate the degradation of exposed masses of rhyolite. The description of these Antrim bosses will be given in [Chapter xlvii]., in connection with the acid eruptive rocks of the Tertiary volcanic series.
2. Structure in the Field
Passing now to the consideration of the lavas as they are built up into the plateaux, we have to note their distinctive characters as individual sheets of rock, and their influence on the topography of the regions in which they occur. Every tourist who has sailed along the cliffs of Antrim, Mull, Skye, or the Faroe Islands is familiar with the singular terraced structure of the great volcanic escarpments which stretch as mural precipices along these picturesque shores. Successive sheets of lava, either horizontal or only gently inclined, rise above each other from base to summit of the cliffs as parallel bars of brown rock with intervening strips of bright green grassy slope.
The geologist who for the first time visits these coast-lines is impressed by the persistence of the same lithological characters giving rise to the same topographical features. He soon realises that the plateaux, so imposingly truncated by the great escarpments that spring from the edge of the sea, are built up essentially of dark lavas—basalts and dolerites—and that fragmental volcanic accompaniments, though here and there well developed, play, on the whole, a quite insignificant part in the structure and composition of these thick piles of volcanic material. Closer examination in the field enables him to ascertain that, regarded as rock-masses, the lavas include four distinct types:—
1st. Thick, massive, prismatic or rudely-jointed sheets, rather more coarsely crystalline and obviously more durable than the other types, inasmuch as they project in tabular ledges and tend to retain perpendicular faces owing to the falling away of slices of the rock along lines of vertical joints. Many rocks of this type are undoubtedly intrusive sheets, and as such will be further referred to in a later chapter. But the type includes also true superficial lavas which show the characteristic slaggy or vesicular bands at their upper and lower surfaces. The mere presence of such bands may not be enough, indeed, absolutely to establish that the rock possessing them flowed at the surface as a lava, for they are occasionally, though it must be confessed rarely, exhibited by true sills. But the rough scoriaceous top of a lava-stream, and the presence of fragments of this surface in the overlying tuff, or wrapped round by the next succeeding lava, sufficiently attest the true superficial outflow of the mass.
2nd. Prismatic or columnar basalts, which, as at the Giant's Causeway and Staffa, have long attracted notice as one of the most striking topographical elements of the plateaux. Columnar structures are typical of the more compact heavy basalts. A considerable variety is observable in the degree of perfection of their development. Where they are least definite, the rock is traversed by vertical joints, somewhat more regular and close-set than those in the dolerites, by the intersection of which it is separated into rude quadrangular or polygonal columns. The true columnar structure is shown in two chief forms. (a) The rock is divided into close-fitting parallel, usually six-sided columns; the number of sides varying, however, from three up to nine. The columns run the whole thickness of the bed, and vary from 8 or 10 to 40 or even 80 feet in length. They are segmented by cross joints which sometimes, as at Giant's Causeway, take the ball-and-socket form. Occasionally they are curved, as at the well-known Clam-shell cave of Staffa. (b) The prisms are much smaller, and diverge in wavy groups crowded confusedly over each other, but with a general tendency upwards. This starch-like aggregation may be observed superposed directly upon the more regular columnar form as at the Giant's Causeway and also at Staffa. Excellent illustrations of both these types may be seen at many points along the sea-cliffs of the Inner Hebrides; the western coast of Skye, the south-west side of Mull, and the cliffs of the island of Canna may be specially cited.
Fig. 259.—Section of scoriaceous and prismatic Basalt, Camas Tharbernish, north shore of Canna Island.
Though generally rather compact, becoming indeed dense, almost vitreous rocks in some sheets, the columnar basalts are often more or less cellular throughout, and highly slaggy along their upper and under surfaces. In some cases, as in that of a prismatic sheet which overlies the rough scoriaceous lava of Camas Tharbernish, in the island of Canna, the rows of vesicles are disposed in lines parallel to the under surface of the sheet ([Fig. 259].)