282 PEOCEEDINGS OP THE GEOLOGICAL SOCIETr. [Apn 5, 



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 FelspatJiic or Augitic. In the former group may be 

 included the pitchstones, trachytes, and porj)hyrites ; in the latter 

 the dolerites, anamesites, and basalts. The fragmental interbedded 

 rocks occur as basalt-tuffs or basalt-breccias. The crystalline intru- 

 sive series is represented by syenites, quartz-porphyries, pitchstones, 

 felstones, dolerites, anamesites, and basalts. The fragmental intru- 

 sive 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, occa- 

 sionally 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 silieated igneous rocks, pitchstone occurs some- 

 what 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 hiUs disrupting and over- 

 lying liassic rocks in Skye and Eaasay. That this syenite belongs 

 to the Tertiary igneous rocks, and may be connected with the vol- 

 canic 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 f. 



The tuffs are comparatively small in quantity. They occur as 

 thin lenticiilar 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 exami- 

 nation of it was that the volcanic rocks had there undergone subsequent meta- 

 morphism, as has happened to the Lias limestones round the Tertiary syenite of 

 Skye. But I propose soon to revisit this interesting district. 



t See Quart. Journ. Geol. Soc. vol. xiv. p. 12 et neg. 



