250 E. B. Bailey 8^ G. W. Grahham—Plagioclase Felspars. 



1906. " A New Palaeoniscid Fish from the Base of the Pendleside Series, near 



Holywell, Flint" : Geol. Mag., Dec. V, Vol. Ill, pp. 556-7, 2 text- 

 figures. 

 " The Fishes of the Old Red Sandstone of Britain." Part II, No. Ill : The 

 Asterolepidfe, pp. 119-30, pis. xxvii-xxxi. Palaeontographical Society, 

 London. 



1907. "The Ganoid Fishes of the British Carhoniferous Formations." Part I, 



PalfEoniscidfe, No. II, pp 87-106, pis. xix-xxiii. Ihid. 

 "Eeport on Fossil Fishes collected by the Geological Survey of Scotland 

 from Shales exposed on the Shore near GuUane, East Lothian " : Trans. 

 Roy. Soc. Edin.. vol. xlvi, pp. 103-17, 2 plates and 2 text-figures. 



1908. " On Fossil Fish Remains collected by J. S. Flett, M.A., D.Sc, from the 



Old Red Sandstone of Shetland " :' ibid., pp. 321-9, 2 plates and 3 text- 

 figures. 



II. — Albitization of Basic Plagiocla.se Pelspaks.^ 



By E. B. Bailey, B.A., and G. W. Grabham, M.A. 



(PLATES X AND XI.) 



SINCE Cathrein ^ in 1883 investigated the nature of saussurite, it tas 

 been well known that albite can arise from the alteration of basic 

 plagioclase felspars. He found that the fine-grained aggregate of 

 minerals known as saussurite has commonly been derived from basic 

 felspar, and that the processes involved in the change have led to little 

 more than a chemical re-arrangement within the limits of the original 

 felspar crystal. The bulk composition of the aggregate thus agrees 

 approximately with that of the felspar from which it has been derived, 

 and the albite which separates out is intermingled with lime-bearing 

 silicates such as epidote, zoisite, and garnet. 



Later, Termier^ has demonstrated the frequent production of albite 

 in the metasomatism of basic plagioclase felspars under conditions 

 which have led to a complete elimination of the original lime. 

 Although the changes investigated by Termier are quite distinct from 

 saussuritization, it is not clear how far they are comparable with what 

 may properly be termed albitization. The albite he refers to is for the 

 most part merely the residue of the original plagioclase, " des matieres 

 etrangeres formant le tiers ou la moitie, ou meme les deux tiers 

 du volume total" (p. 186). He does, however, deal with examples 

 where an actual gain of soda (p. 190) appears to have resulted. Here 

 there would seem to have been a partial replacement of the original 

 lime-felspar by soda-felspar, and this change, which may fairly be 

 designated albitization, is now known to occur in several widely 

 separated localities. Duparc & Pearce,'' for instance, have described 



1 By permission of the Director of H.M. Geological Survey. 



2 Zeit. Erist., 1883, Band vii, p. 234. 



2 " Sur I'elimination de la Chaux par metasomatose dans les roches eruptives 

 basiques de la region du Pelvoux": Bull. Soc. Geoi. France, 1898, vol. xxvi, 

 pp. 165-92. 



* " Sur les andesites et les basaltites albitisees du Cap Marsa " ; Cmnptes Eendus, 

 1900, vol. cxxx, p. 96. Two references to the replacement of potash-felspar by 

 secondary albite may here be inserted : F. Becke, " Zur Physiograpliie der 

 Gemengteile der Krystallizen Schiefer," Wien K. K. Akad. Wiss. DenJcschr., 

 1906, vol. Ixxv, p. 28 ; and A. C. Spencer, " The Treadwell Ore Deposits, 

 Douglas Island," Bull. U.S. Geol. Surv., 1904, No. 259, p. 74. 



