518 Proceedings of the Royal Irish Academy. 



much more than ten years have elapsed since the mineral was first 

 observed by Bonney,' in granite from Socotra ; and it was not till some 

 years later that Sauer- first recognised in it a new member of the 

 hornblende series, and gave it its present name. Its occurrence in the 

 British Isles was first made known by Bonney^ and Harker,* who 

 described it from Mynydd Mawr, in North Wales. A description of 

 the Ailsa Craig rock, which has furnished fragments to glacial deposits 

 on both sides of the Irish Sea,^ and the Isle of Man,® we owe to 

 Teale.^ The most recent reference to riebeckite is by Brogger,® who 

 indicates the close relationship of this mineral and crocidolite to arfved- 

 sonite. The .association of crocidolite and riebeckite had been earlier 

 pointed out by Lacroix.^ 



A summary of the results of the measurements I have made is 

 given, antea (p. 516), and I now proceed to offer some additional 

 details. 



Fig. 2. 

 All the crystals afforded by the pebble from Portrane present 



1 Bonney, Phil. Trans. Eoy. Soc, vol. i., p. 273, 1883. 

 - Sauer, Zeits. d geol. Ges., vol. xl., p. 138, 1888. 

 •* Bonney, Mineralogical Magazine, vol. viii., pp. 103, 169, 1888. 

 ■* Harker, Geological Magazine, p. 455, 1888. 



•' Lomas, Eeport British Assoc, p. 707, 1892. Cole and Sollas {loc. eit.). 

 6 Cole, Mineralogical Magazine, vol. ix., p. 222, 1891. Kendall, Jomn. Isle of 

 Man Nat. Hist, and Antiquarian Soc, vol. i., p. 410, 1894. 

 ' TeaU, Mineralogical Magazine, vol, ix., p. 219, 1891. 



^ Brogger, Die Eruptivgesteine des kristianiagebictes, p. 38. Kristiania, 1894. 

 ^ Lacroix, Comptes Eendu, vol. cix., p. 39. 



