VII.—On the occurrence of Sillimanite Gneisses in Central Anglesey
and the irregular-shaped pieces are surrounded by felsitic matter. The microscopic examination showed that the felsite contained a great number of -very small colourless crystals, which sometimes show distinct hexagonal outlines and contain minute black inclosures. These crystals accumulate round the pieces of clayslate, and decrease in number the further away from the slate one examines the slice. I have for some time been very much puzzled as to what this mineral could be. By dissolving a
... By dissolving a portion of the rock in hydrofluoric and hydrochloric acid a white powder remained, which under microscopic examination proved to consist solely of the crystals in question. By analyzing these I found nothing but alumina and very small traces of iron-oxide, probably due to the minute black inclosures, which I suppose to be magnetite. I then tried to determine the hardness. I put a little of the powder between two cleavage-flakes of topaz, which after a little rubbing soon lost their lustre and became perfectly dull. There can be no doubt, therefore, that the mineral in question is corundum ; and also, considering the manner in which it occurs, that it is not an original constituent of the felsitic porphyry. My belief in regard to the origin of this corundum is this, that the eruptive rock, when the eruption took place, partly dissolved and absorbed the broken pieces of the clay-slate, and that afterwards in this felsitic matter, which was then supersaturated with alumina, the crystallization of the corundum took place on cooling. We are aware of the fact that corundum as a product of contactmetamorphism is very rare, and has, indeed, only lately been described in one or two cases, which, however, are quite different from the occurrence on Dartmoor. I therefore thought it would be of some interest to record this discovery, all the more as it is, so far as I know, the first instance of the occurrence of corundum in this way in the British Islands.