A gabbro-granodiorite contact in the Slieve Gullion area and its bearing on Tertiary petrogenesis

Dr. Reynold's excellent paper is welcome as extending to British Isles the recognition of certain phenomena that have often been overlooked in describing the intrusion of basic into acid igneous rocks. Dr. Richey has expressed the hope that " the author's discoveries will be followed up in other cases, perhaps from other countries ", so it may be well to point out that similar observations have been recorded from Rhodesia and South Africa generally over a period of more than thirty years. The first of these appeared in the Geological Magazine for January, 1911, under the title of " Observations on some basic dykes and their bearing on certain problems of rock genesis ". The Irish rocks show the initial stages of the processes there described, their features being similar to those of the smaller of the two intrusions into the Matopo granite referred to in my paper. The fact that the quartz of the invaded rock is " usually surrounded by a rim of fine-grained material" is noted, and is illustrated in fig. 2, which also shows the clouding, motting, and granulation or "reticulation" of the felspars due to thermal alteration. A minor difference is that " between the quartz and felspar grains there is always a reaction rim 1/10 mm. broad ". In the larger intrusion at Kahlele's, which is more gabbro-like than Dr. Reynolds's rock, though I still prefer to call it a dolerite, secondary micropegmatite is developed on a large scale and in extreme cases forms