opal vary considerably from those of the pearlstone both in nature 

 and structure, and the glassy base of the former is almost 

 completely chalcedonic. Various types of these cavities were 

 described in the course of the lecture, and numerous specimens 

 were exhibited showing the different varieties of opal found in the 

 district. One of the specimens shown was the rather rare and 

 interesting variety known as the " hydrophane," which is white 

 and opaque when dry, but on immersion in water it absorbed 24 

 per cent, of its own weight of the latter, becoming transparent and 

 showing a play of colours like precious opal. In conclusion, the 

 lecturer discussed briefly the probable origin of the opal deposits 

 at Sandy Braes, comparing the cavities in the porphyry with the 

 well-known " lithophyse " structure of some foreign lavas. 



An animated discussion followed the lecture, in which the 

 Chairman, Messrs. Thomas Dewhurst, A.R.CSc , James Orr, 

 H. L. Orr, and George Donaldson took part. Mrr Strachan was 

 highly complimented on his research work on these local deposits, 

 and in his reply he ably defended the theoretical views he had 

 advanced. 



" LAND SHELL DEPOSIT ON SANDHILLS, HORN HEAD." 



A joint meeting of the Zoological and Geological Sections 

 was held on the evening of the 3rd March — the Vice-President, 

 Mr. W. H. Gallway, in the chair — when a short description of 

 some recently discovered " rainwash " deposits of land shells in 

 the sand hills at Horn Head, County Donegal, was given by 

 Mr. Robert Welch, M.R.I. A. The lecturer pointed out the 

 difference between the well-known " pockets " of land shells 

 collected by the wind, occurring on dunes, and deposits formed 

 during heavy rains in cases where these dunes encroach on 

 steep hillsides. In both instances these "pockets" collect in 

 hollows, and are usually covered up with blown sand. If the sand 

 dunes are in the process of formation, the shells form local 



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