[P5831] 
XLIX. 
ON THE OCCURRENCE OF CASSITERITE IN THE TERTIARY 
GRANITE OF THE MOURNE MOUNTAINS, CO. DOWN. 
By HENRY J. SEYMOUR, B.A., F.G.S. 
[Read June 18; Received for Publication Junz 20; Published SrptEmBer 13, 1902. ] 
THIs communication is intended to put on record the occurrence 
of Cassiterite in a new locality in Ireland. The writer, while 
lately examining (in connexion with a revised Catalogue of Irish 
Minerals which he has in preparation) a collection of minerals 
from the Mourne Mountain district, made by Dr. E. A. Letts, 
Professor of Chemistry at the Queen’s College, Belfast, had his 
attention directed by the latter to some specimens of beryl from 
Slieve-na-miskan, on which were implanted some very minute 
brownish-black crystals. These Dr. Letts had noticed when he 
collected the specimens, but had taken no further steps to identify. 
The writer being unable, through lack of facilities at the time, to 
diagnose the species, sent the material to the British Museum, 
where the mineral in question was determined: by Mr. L. J. 
Spencer to be Cassiterite. 
The crystals so identified are almost of microscopical dimen- 
sions, but more recently some good crystals, visible to the unaided 
eye, have been found amongst the material originally collected by 
Dr. Letts. They occur for the most part on the beryls (aqua- 
marines), but are also found in the small drusy cavities in their 
immediate vicinity. The matrix is a much altered granite, in 
which the felspars are completely kaolinized, or washed away, and 
contains also secondary silica, so that the rock is much more 
quartzose than usual, and sometimes presents a spongy character. 
This occurrence of Cassiteritein an undoubted Tertiary granite 
is, if not unique in the British Isles, at all events of very great 
interest, as tending to show the comparatively recent origin of 
some mineral lodes. 
SCIEN. PROC., R.D.S., VOL. IX., PART V. 2Z 
