200 [Proc. B.N.F.C.. 



must have much truth embedded in it. Science, with its 

 unwritten records, aided the investigation between fact and fable. 

 They had in Ireland both the texts and unwritten records, Europe 

 having only the unwritten records. Celtic history was the coming 

 field of study. 



The paper, which was listened to throughout with marked 

 interest, was spoken to by the Chairman, Miss Andrews, Mrs. 

 Hobson, Messrs. J. M. Dickson, Wm. Gray, M.R.I. A., D. Elliott, 

 M.A., and Dr. Clarke Robinson, M.A. 



" OPAL DEPOSITS OF SANDY BRAES : THEIR FORMATION 

 AND ORIGIN." 



At a meeting of the Geological Section held on Wednesday, 

 evening, 24th February, in the Museum, a lecture was given by 

 Mr. James Strachan on " The Opal Deposits of Sandy Braes : 

 their Formation and Origin." The chair was occupied by Mr. 

 W. J. C. Tomlinson, and there was a representative attendance of 

 members. Mr. Strachan, in the course of his address, said the 

 opal deposits of Sandy Braes, County Antrim, are, like the 

 rhyolitic lavas in which they are found, quite unique in the 

 geology of the British Islands. The prevailing type of rock at 

 this locality is a yellow fluidal pearlstone, with typical perlitic and 

 flow-structures, variegated by bands and masses of a puce-brown 

 colour, showing flow-structures on a large scale. In this lava 

 occur cavities, varying in size from 1 to 20 centimetres, containing 

 opal and hyalite. In places the latter passes into chalcedony. 

 Each cavity is surrounded by a wall or layer of purple-red 

 porphyry or pitchstone, 1 to 7 centimetres in thickness. These 

 hollow " bombs " of porphyry, more or less completely filled with 

 opaline minerals, are found upon the surface at Sandy Braes, 

 where they have been weathered out from the pearlstone. By 

 digging into the fresh rock fine specimens may be obtained in 

 situ. The constituent minerals of the porphyry surrounding the 



