260 E. D. Wellbtiyn—On Rhadiuichthys. 



In some isolated cases the blue ampliibole shows wavy extinction- 

 under crossed nicols, due apparently to a gradual change in chemical 

 composition during its formation. It was found impracticable to 

 isolate material for chemical analysis., but there can be no reasonable 

 doubt that it belongs to the soda amphibole group of which 

 riebeckite and arfvedsonite are typical examples. 



The occurrence of a secondary blue amphibole under conditions 

 exactly similar to those just detailed above has been described by 

 Dr. Whitman Cross ^ in the case of an allied rock from Colorado. 

 A. C. Lane^ also records a similar occurrence in a syenite from 

 Michigan. More recently J. S. Flett^ notes the presence of 

 a secondary blue amphibole in cracks in crystals of hornblende in 

 one of the lamprophyre dykes of the Orkney Islands. The mineral 

 from the Co. Down corresponds so closely in every particular witli 

 those from the two American localities aboved cited, that the writer 

 has little hesitation in referring it to the same species. The mineral 

 described by Cross has been called crossite by Ch. Pelache/ who 

 also has described a species with identical optical projjerties. 

 Dr. Cross, however, in a later publication ^ states that the mineral 

 in question is considered to be arfvedsonite by Dana,'' Broggex','' and 

 Kosenbusch.'' It may be noted further that Iddings, in his trans- 

 lation ® of Eosenbusch's work, describes the secondary amphibole of 

 Cross in the context between glaucophane and riebeckite, which 

 position the at any rate optically similar species of Pelache ^° 

 occupies by virtue of its intermediate chemical composition. Eosen- 

 busch" notes the occurrence of a blue amphibole in a minette from 

 Wachenback in the Vosges, the rock belonging to the same group as 

 the Co. Down specimen. 



VI. — On Rhadixichthys monensis, Egerton, and its Distribution 



IN THE Yorkshire Coalfield. 



By Edgar D. Wellburn, L.E.C.P., F.G.S., F.K.I.P.H., etc. 



THIS species was first described by Sir P. De M. Grey Egerton^- as 

 Palceoniscus monensis, in 1850, from detached scales from the 

 Coal-measures of Anglesey. Subsequently, Dr. E. H, Traquair^* 

 proved that the proper position of the fish was in his genus Rhadin- 

 ichthys. Up to this date only detached scales had been known, but 

 in the paper just mentioned Dr. Traquair described specimens 

 showing many points in the structure of the fish which had been 



1 Amer. Journ. Sci., vol. xxxix (1890), p. 359 et seq. 



* Amer. Joum. Sci., vol. xlii (1S91), p. 505. 



5 Trans. Roy. Soc. Edinbirrgli, vol. xxxix (1900), pt. iv, p. 884. 



^ Bull. Dept. Geol. Univ. California, 1894, p. 181 et seq. 



5 16tli Ann. Rep. U.S. Geol. Survey, 1894-95, pt. ii, p. 30. 



^ System of Mineralogy, 6th edition", 1892, p. 402. 



' Die Gesteine der Grorudit-Tinguait-Serie, 1894, p. 33. 



8 Die mikro. Phys. der petro. wichtigen Min., 3rd ed., 1892, p. 567. 



" Translation of the last work by Iddings, 1898. p. 270 



10 Loc. cit., p. 189. 



'• Mikro. Phys. d. raassigen Gesteine, 1896, p. 510. 



1" Quart. Journ. Geol. Soc, vol. vi (1850), p. 5. 



15 Proc. Roy. Phys. Soc. Edinb., vol. iv (1878), p. 241. 



