49-i Edward Greenly — Sillimanite Gneisses in 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 some- 

 times show distinct hexagonal outlines and contain minute black 

 inclosures. These crystals accumulate round the pieces of clay- 

 slate, 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 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 contact- 

 metamorphism 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. 



VII. — On the occurrence of Sillimanite Gneisses in Central 



Anglesey. 1 

 By Edward Greenly, F.G-.S. 



IN the course of some traverses of the central region of Anglesey 

 recently made, preparatory to mapping it out in detail, I 

 observed a few facts which, in spite of the evident complexity of the 

 structures, can, I think, be safely described before completion of the 

 map, and are of some interest in connection with other recent 

 researches in gneissose areas. 



The region in question (see descriptions by Sir A. C. Ramsay, 

 Dr. Callaway, the Rev. J. F. Blake, Sir A. Geikie, and others) extends 

 across the island in a north-easterly direction from Llanfaelog to 

 near Llanerchymedd. 



To the south-west of the Holyhead Boad granite appears to occupy 

 the greater part of the surface, but to the north-east a complex system 



1 An abstract of this paper was 'read before the British Association, Liverpool, 

 September, 1896. 



