230 ME. J. J. H. TEALL ON A PHOSPHATIZED [May 1 898, 



18. A Phosphatized Teach yte from Clippeeton Atoll (Noetheen 

 Pacific). By J. J. H. Teall, Esq., M.A., F.R.S., Y.P.G.S. 



(Eead March 9tli, 1898.) 



[Plate XXIII.] 



The following remarks are based on five rock- specimens from 

 Clipperton Atoll, referred to by Admiral Wharton in his description 

 of that island. 



One is a dark brown rock composed of porphyritic crystals of 

 glassy felspar and a compact groundmass ; the others are white or 

 cream-coloured, and in some of these the same porphyritic felspars 

 as in the first-mentioned rock may be observed. The examination 

 of a series of microscopic sections shows that the rocks are all more 

 or less altered trachytes. 



The brown rock is the least altered. It consists of phenocrysts 

 of sanidine (PI. XXIII, fig. 1) set in a groundmass of microlitic 

 felspars and brown interstitial matter. 'No ferro-magnesian minerals 

 are recognizable. The conspicuous phenocrysts of sanidine are, as a 

 rule, comparatively free from inclusions ; but the rock contains also 

 some patches of felspar, which are so crowded with inclusions of a 

 brown substance that the felspar-material forms only a small portion 

 of the compound mass. 



An analysis of this rock yielded the following somewhat sur- 

 prising result : — 



SiO, 54-0 



PA 8-4 



AI263 17-9 



FeA 4-4 



CaO 1-4 



K,0 4-5 



Na^O 50 



Loss on ignition 3"8 



99-4 



The phosphoric acid is present in the brown substance, which 

 evidently represents, in a more or less altered form, the interstitial 

 matter of the original trachyte. 



The microscopic sections of the white or cream-coloured rocks all 

 show the structure of a trachyte. In some specimens the pheno- 

 crysts of sanidine have been more or less preserved, while the 

 groundmass has been replaced by isotropic secondary material ; in 

 others the phenocrysts have disappeared, and their places have been 

 wholly or partially filled up with the same secondary substance. 

 It is evident, therefore, that a trachyte has been attacked by some 

 chemical process, and that a pseudomorph of the rock has been 

 formed. 



