The Ancient Volcanoes of Great Britain, Volume 2 (of 2)

by Archibald Geikie

By far the greater number of the dykes of the Tertiary volcanic series belong to the first group, and it is these more especially which will be discussed in the present and the following Chapter. As, however, the andesitic group is intimately linked with the basaltic it will be here included with them.

1. Basalt, Dolerite and Andesite Dykes.—To the field-geologist, who regards merely their external features, the Tertiary dykes present a striking uniformity in general petrographical character. They vary indeed in fineness or coarseness of texture, in the presence or absence of porphyritic crystals, amygdales, glassy portions and other points of structure. But there is seldom any difficulty in perceiving that they generally belong to one or other of the types of the basalts, dolerites, diabases or andesites. This sameness of composition, traceable from Yorkshire to Skye and from Donegal to Perthshire, is one of the strongest arguments for referring this system of dykes to one geological period. At the same time, there are enough of minor variations and local peculiarities to afford abundant exercise for the observing faculties alike in the field and in the study, and to offer materials for arriving at some positive conclusions regarding the geological processes involved in the uprise of the dykes.

There appears to be reason to believe that, when the petrography of the dykes is more minutely studied, marked differences of material will be found to denote distinct periods of eruption. Already Mr. A. Harker of the Geological Survey, who is engaged in mapping the interesting and complicated district of Strath in Skye, has observed that the dykes which are older than the great granophyre bosses of that tract may be distinguished from those which are later than these protrusions. The older basic dykes are not conspicuously porphyritic, are frequently marked by a close-grained margin or even with a veneer of basalt-glass, sometimes have an inclination of as much as 45°, are occasionally discontinuous, and not infrequently branch or send out veins. The younger dykes, on the other hand, as will be more particularly noticed in the following chapter, are distinguished by the frequent and remarkable character of their porphyritic inclusions, by the presence of foreign fragments in them, by the greater perfection of their jointing, and by their seldom departing much from the vertical.[163] They are likewise often markedly acid in composition, including such rocks as granophyre, felsite and pitchstone.

[163] In the Blath Bheinn group of gabbro-hills, however, it is the youngest dykes which have been found by Mr. Harker to possess the lowest hade.

(1) External Characters.—As regards the grain of the rock, every gradation may be found, from a coarsely crystalline mass, in which the component minerals are distinctly traceable with the naked eye, to a black lustrous basalt-glass. Each dyke generally preserves the same character throughout its extent. As a rule, broad and long dykes are coarser in grain than narrow and short ones. For the most part, there runs along each side of a dyke a selvage of finer grain than the rest of the mass. This marginal strip varies in breadth from an inch or less up to a foot or more, and obviously owes its origin to the more rapid chilling of the molten rock along the walls of the fissure. It usually shades away inperceptibly into the larger-grained inner portion. Even with the naked eye its component materials can be seen to be more finely crystalline than the rest of the dyke, though where dispersed porphyritic felspars occur they are as large in the marginal strip as in any other part of a dyke, for they belong to an earlier period of crystallization than the smaller felspars of the groundmass and were already floating in the magma while it was still in a molten state.

This finer-grained external band, so distinctive of an eruptive and injected rock, is of great service in enabling us to trace dykes when they traverse other dykes or masses of igneous rock of similar characters to their own. When one dyke crosses another, that which has its marginal band of finer grain unbroken must obviously be the younger of the two.

Fig. 235.—Plan of basalt-veins with selvages of black basalt-glass, east side of Beinn Tighe, Isle of Eigg.

But in many examples in the south of Scotland, Argyleshire and the Inner Hebrides, the fineness of grain of the outer band culminates in a perfect volcanic glass. Where this occurs, the glass is usually jet black, more rarely greenish or bluish black in tint, and varies in thickness from about a couple of inches to a mere varnish-like film on the outer face of the dyke, the average width being probably less than a quarter of an inch ([Fig. 235]). On their weathered surface these external glassy layers generally present a pattern of rounded or polygonal prominences, varying up to four or five lines or even more in diameter, and separated by depressions or narrow ribs. The transition from the glass to the crystalline part of the marginal fine-grained strip is usually somewhat abrupt, insomuch that on weathered faces it is often difficult to get good specimens, owing to the tendency of the vitreous portion to fly off when struck with the hammer. The glass doubtless represents the original condition of the rock of the dyke. It was suddenly chilled and solidified by contact with the cold walls of the fissure. Inside this external glassy coating, the molten material could probably still move, and had time to assume a more or less completely crystalline condition before solidification. Not infrequently the glass shows spherulitic forms, visible to the naked eye, and likewise a more or less distinctly developed perlitic structure. These features, however, are best studied in thin sections of the rock with the aid of the microscope, as will be subsequently referred to.

In some dykes, the glass is not confined to the edges, but runs in strings or broader bands along the central portions, or has been squeezed into little cavities like steam-holes or into minute fissures. One of the most remarkable examples of this peculiarity occurs in the well-known dyke of Eskdale, which runs for so many miles across the southern uplands of Scotland.[164] This dyke throughout most of its course is a crystalline rock of the andesitic type. At Wat Carrick, in Eskdale, it presents an arrangement into three parallel bands. On either side, a zone about eight feet broad consists of the usual crystalline material. Between these two marginal portions lies an intercalated mass 16 to 18 feet broad, of a very compact and more or less vitreous rock. The demarcation between this central band and the more crystalline zones of the outside is quite sharp, and the two kinds of rock show a totally distinct system of jointing. There can, therefore, be little doubt that the glassy centre belongs to a later uprise than the outer portions, though possibly it may still have been included in the long process of solidification of one original injected mass of molten material. If the marginal parts adhered firmly to the walls, the centre, which with its band of vesicles seems often to have been a line of weakness, might be ruptured and subsequent intrusions would find their way along the rent. Examples of this splitting of dykes with the intrusion of later eruptive Material will be cited in later pages.