The superficial outbursts of volcanic action during Tertiary time in Britain are represented by a comparatively small variety of rocks. These consist almost wholly of basalts, but include a number of less basic rocks which may be classed as andesites. Many andesitic sheets, like the andesitic dykes, have been intruded into the basalts, and are really sills.
Besides the lavas of the basaltic-plateaux there are intercalated deposits of tuffs and breccias and large masses of agglomerate. A brief notice of the general petrography of the various constituents of the plateaux and their mode of occurrence will here be given. The intrusive bosses which have disrupted the superficial lavas will be discussed in subsequent chapters.
i. LAVAS
1. Petrographical Characters
(a) Basalts and Dolerites.—In external characters these rocks range from coarsely crystalline varieties, in which the constituent minerals may be more or less readily detected with the naked eye or a field-lens, to dense black compounds in which only a few porphyritic crystals may be megascopically visible. One of their characteristic features is the presence of the ophitic structure, sometimes only feebly developed, sometimes showing itself in great perfection. Many of the rocks are holocrystalline, but usually show more or less interstitial matter; in others the texture is finer, and the interstitial matter more developed; in no case, as far as I have observed, are there any glassy varieties, which are restricted to the dykes and sills, though in some of the basalts the proportion of glassy or incompletely devitrified substance is considerable. The felspars are generally of the characteristic lath-shaped forms, and are usually quite clear and fresh. The augite resembles that of the dykes, occurring sometimes in large plates that enclose the felspars, at other times in a finely granular form. Olivine is frequently not to be detected, even by green alteration products. Magnetite is sometimes present in such quantity as to affect the compass of the field-geologist. Porphyritic varieties occur with large felspar phenocrysts; but such varieties are, I think, less frequent among the plateau-rocks than among the dykes. They are well developed in the west part of the island of Canna, and have been described from the Faroe islands. Occasionally the plateau lavas are full of enclosed fragments of other rocks which have been carried up in the ascending magma.
(b) Andesites and Trachytes.—Probably the majority of these rocks where they occur intercalated between the basalts of the plateaux are, as already remarked, intrusive sheets rather than true lavas. But they have also been poured out intermittently among the basalts and dolerites. The most extensive development of lavas which are readily distinguishable from the group of plateau-basalts, and must be placed in the present series, occurs in the island of Mull. These rocks form part of a group of pale lavas which overlie the main mass of the plateau-basalts, and cap the mountain Ben More, together with several of its lofty neighbours. They are interstratified with true ophitic dolerites, and basalts showing characteristic granular augite. They are not so heavy as the ordinary plateau-lavas, their specific gravity ranging from 2·55 to 2·74. Externally they are light grey in colour and dull in texture, sometimes strongly amygdaloidal, sometimes with a remarkable platy structure, which, in the process of weathering, causes them to split up like stratified rocks. In some of their amygdaloidal varieties the cells are filled with epidote, which also appears in the fissures, and sometimes even as a constituent of the rock.
Specimens from this "pale group" of Ben More, when examined in thin slices under the microscope, were found by Dr. Hatch to consist almost wholly of felspar in minute laths or microlites, but in no instance sufficiently definite for satisfactory determination. In one of them he observed that each lath of felspar passed imperceptibly into those adjacent to it; the double refraction being very weak, and the twin-striation, if present, not being traceable.[220] More recently my colleague, Mr. W. W. Watts, has looked at some of the same slides. He is disposed to class the rocks rather with the trachytes than the andesites. He remarks that "in the apparent holocrystalline character, the size and shape of the felspars, the sort of damascened appearance in polarized light, the finely scattered iron-ores and the presence of a pale green hornblende, possibly augite, in small, often complex, grains, these rocks much resemble the Carboniferous trachytes of the Garlton Hills in Scotland."
[220] In the course of my investigations I have had many hundreds of thin slices cut from the Tertiary volcanic rocks for microscopic determination. These I have myself studied in so far as their microscopic structure appeared likely to aid in the investigation of those larger questions of geological structure in which I was more especially interested. But for further and more detailed study I placed them with Dr. Hatch, who submitted to me the results of his preliminary examination, and where these offered points of geological import I availed myself of them in the memoir published in 1888 in the Transactions of the Royal Society of Edinburgh. I have retained most of these citations in their place in the present volume, and have supplemented them by notes supplied to me from fresh observations by Mr. Watts and Mr. Harker. Professor Judd, in a series of valuable papers, has discussed the general petrography of the Tertiary volcanic rocks (Quart. Jour. Geog. Soc. vols. xxxix. xli. xlii. xlvi. xlix.)
One of the most interesting lavas of the Tertiary volcanic series is the "pitchstone-porphyry" of the Scuir of Eigg. This rock, the latest known outflow of lava in any of the volcanic areas of Britain, was formerly classed with the acid series. Microscopical and chemical analyses prove it, however, to be of intermediate composition, and to be referable to the andesites or dacites. It is more particularly described in [Chapter xxxviii].
Professor Judd, collecting the andesitic rocks as a whole (both lavas and sills), has grouped them into amphibole and mica-andesites, and pyroxene-andesites.[221] The thick lumpy and non-persistent sheets of these rocks sometimes found near the centres of protrusion of the gabbros and granophyres are probably sills.