THE general geology of this district has been described in the foregoing paper. The igneous petrology will now be described. The rocks will be taken in the following order: -I. Fragmental rocks of the vents. II. The intrusions in the vents. III. The lavas of Auchineden Hill. IV. The E.-W. dyke. I. FRAGMENTAL ROCKS OP THE VENTS. These vary from coarse, pebbly agglomerates composed of fragments of lavas and sandstones to fine-grained grey, green, purplish, or red-coloured tuffs. It will be

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... ent as a typical example to describe the material filling the Little Caldon neck. This is a medium-grained variety which in thin section is seen to consist of fragments of porphyritic olivine basalt lava, large detached crystals of serpentinised olivine, and abundant detrital quartz, mostly well rounded, but occasionally angular and subangular. There are also a few broken crystals of fresh orthoclase. Both the latter con stituents are obviously derived from the adjacent sandstones. In another slide of the same rock occur fragments of yellow, partially hornfelsed shale, and pieces of another type of lava, very fine grained and porphyritic with felspar, not olivine. A thin section of a tuff from the Knockvadie neck shows a compact groundmass of magnetite, containing scattered laths of plagioclase, numerous rounded areas filled with calcite, and occasional grains of quartz derived from the adjacent sandstones.