In the Glas Eilean vent occur agglomerates, acid veins, and an acid dyke. Petrographic studies lead to the conclusion that the matrix of the agglomerates, acid material from the veins, and the acid rock from the dyke all represent intruded tuffaceous material, which is called “tuffisite”. The acid-veining is of non-dilational type, where fragments of different rocks, shapes and sizes are cemented together by intruded tuffaceous material. The occurrence of acid-veins in the Glas Eilean vent containing agglomerate demonstrates that net-veining is essentially a volcanic phenomenon originating at shallow depth. The peculiar shape of the acid-dyke and the inclusions of the large number of vent-breccia fragments suggest that the dyke was formed as an intrusive tuff by hot gases charged with magma. The intimate association in the tuff of two contrasting glasses, viz. acid and basic, provides evidence of liquid immiscibility. The Glas Eilean vent was initiated by a violent explosion which was followed by gas-streaming as envisaged by Reynolds. This brought about the emplacement of the tuffisite. The rounded nature of some fragments suggests the role of sandblast in a fluidized system.