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



Since tlie above was written my attention has beeD called to an 

 important series of papers ^ by M. Armand Gautier on the phosphatic 

 deposits of the ' Grotte de Minerve.' 



This cavern is situated in the valley of the Cesse, in Herault, at 

 the junction of the Xummulitic Limestone and Devonian. The walls 

 and roof are of limestone ; the floor is formed of a Quaternary 

 deposit, containing mammalian remains and resting on the 

 Devonian. At a depth of 3 or 4 metres from the surface a yellowish 

 rock is met with, containing 50 per cent, of the phosphates of lime 

 and alumina. During his excavations in this cavern M. Gautier 

 found a vein of a white, pasty substance, composed almost entirely 

 of a hydrated phosphate of alumina (P.^O^, Al^Og, 7 H^O), for 

 which he proposed the name ' minervite.' He considers tiiat this 

 substance has been formed by the action of ammonium phosphate 

 arising from the decomposition of organic matter, on clay or hydr- 

 argillite. In support of this view he shows that precipitated alumina 

 can be wholly converted into a phosphate having the composition of 

 minervite, by the action of a solution of ammonium phosphate, and 

 that kaolin can, in the same way, be partiallj^ changed into a 

 similar phosphate. He refers to the deposits of Alta Yela, Hedonda, 

 and Commandeur (French Guiana), and concludes that they have 

 been formed in a similar manner. 



Owing to the kindness of Mr. J. Hort Player, I have obtained 

 specimens of the phosphates of Eedonda and Connetable (French 

 Guiana). Those from Kedonda are brown or buff-coloured porous 

 rocks, some of which clearly show the structure of an audesite. 

 There can be no doubt that some, if not all, of the Eedonda 

 phosphate has been formed from an andesite in the same manner as 

 that of Clipperton has been formed from a trachyte. 



The specimens from Connetable, a small island near Cayenne, do 

 not show igneous structure. They are whiter than the specimens 

 from Eedonda, and closely resemble some portions of the Clipperton 

 rocks in which the igneous structures have disappeared. 



We may, therefore, conclude that the formation of phosphates of 

 alumina and iron is by no means an uncommon occurrence when 

 guano is deposited on igneous rocks. 



EXPLANATION OF PLATE XXIII. 



Fig. 1. Altered trachyte from Clipperton AtoU, showing phenocrysts ot 

 sanidine set in a groundmass of microlitic felspars and brown 

 interstitial matter. In the central lower portion of the slide is an 

 example of the felspar crowded with brown inclusions. 

 2. Highly altered trachyte, showing the replacement of felspar by phos- 

 phate with concretionary structure, and the groundmass replaced 

 by a similar material, but without concretionary structure. 



^ ' Formation des Phosphates naturels d'Alumine et de Fer,' Comptes Eendus, 

 Acad. Sci. Paris, vol. cxvi (1893) p. 1491. See also other papers in the same 

 volume on pp. 928, 1022, and 1171. 



