This general geological classification admits of and requires further subdivisions, according to the petrographical distinctions of the rocks. Thus the Tertiary volcanic rocks which occur as crystalline interbedded sheets may be grouped, according to their mineralogical composition, as Felspathic or Augitic. In the former group may be included the pitchstones, trachytes, and porphyrites ; in the latter the dolerites, anamesites, and basalts. The fragmental interbedded rocks occur as basalt-tuffs or basalt-breccias. The crystalline intrusive series is represented by syenites, quartz-porphyries, pitchstones, felstones, dolerites, anamesites, and basalts. The fragmental intrusive series is shown by necks of basalt- agglomerate.
The dolerites, anamesites, and basalts form the great mass of the Tertiary volcanic rocks of Britain. They occur in vast plateaux, as in Antrim and the Inner Hebrides, also abundantly as dykes, veins, and intrusive sheets. They vary in texture from a coarse crystalline aggregate to fine black basalt, which, in turn, shades into the glassy variety known as tachylite. In interbedded sheets they are columnar or jointed, often amygdaloidal, and then full of zeolites. Closely related to these, and possibly a metamorphosed variety of them, are some rocks in which diallage occurs in place of augite*. Much less abundant are some pale grey rocks, sometimes amygdaloidal, occasionally very porphyritic, composed of a dull plagioclase base, with striated felspar crystals, and for which porphyrite is perhaps the most fitting name. They occur in interbedded sheets in Mull and Eigg. Of the more highly silicated igneous rocks, pitchstone occurs somewhat rarely, and always in the form of veins, except in the old coulee of the Scur of Eigg, to be described in this paper. Felstone and quartziferous porphyry occur in veins and intruded masses. Syenite is found in veins, and also as huge hills disrupting and overlying liassic rocks in Skye and Raasay. That this syenite belongs to the Tertiary igneous rocks, and may be connected with the volcanic eruptions of the great basalt-plateaux, I hope to show in a future paper. A rock which has been called a trachyte-porphyry occurs in Antrim. I may add that around the syenite-hills of Skye, and possibly also in Mull, there has been developed a local but well- marked metamorphism of the surrounding rocks†.
The tuffs are comparatively small in quantity. They occur as thin lenticular layers between the sheets of dolerite forming the great plateaux, and sometimes, as at Ardtun Head, Mull, and in Antrim, contain recognizable remains of land-plants. In Mull also they are sometimes associated with local beds of black cherry-coal, not distinguishable by any external character from the ordinary fuel of our coal-fields. Necks of agglomerate are of still rarer occurrence. Between the sheets of dolerite thin irregular layers of
- These are seen to the south-east of Ben More, in Mull, and seemed to me to
be a continuation of beds which, further west, were ordinary dolerites. In that area also masses of syenite occur ; and the impression conveyed by a hasty examination of it was that the volcanic rocks had there undergone subsequent metamorphism, as has happened to the Lias limestones round the Tertiary syenite of Skye. But I propose soon to revisit this interesting district.
† See Quart. Journ. Geol. Soc. vol. xiv. p. 12 et seq.
