238 Reports and Proceedings — Minercdogical Society. 



belonged, and the paper now about to be read gives the result of the study of 

 this minute specimen by Dr. Washington. 



The following comraunication was read : — 



" The Composition of Eockallite." By Dr. Henry S. Washington, 

 For. Corr. G.S. 



The paper centres around a detailed chemical analysis made by 

 the author of the type-material of rockallite, an igneous rock of 

 exceptional chemical and mineralogical composition. 



A petrographical account is given of the rock, with special reference 

 to the influence of the constituent minerals upon the bulk-analysis. 



Rockallite has a fine-grained granitic structure, and is composed of 

 about equal amounts of colourless quartz, alkaline felspar, and soda- 

 pyroxene. The pyroxene is of two kinds, a bright grass-green segirite 

 and a pale yellowish-brown acmite. Some zircon is present, but not 

 to more than a tenth of 1 per cent of the rock. 



A chemical analysis has been made with the greatest care, according 

 to the methods advocated by Hillebrand and the author, zirconia and 

 the rare earths being especially looked for. The main results of the 

 analysis confirm those of Makins, but several new points of interest 

 have presented themselves. The outstanding features of the rock 

 appear to be the high percentages of silica, ferric oxide, and soda, and 

 the low percentages of alumina, ferrous oxide, magnesia, lime, and 

 potash. The special interest of the new analysis, however, lies in the 

 detection of zirconia and cerium oxide in unexpectedly large amounts, 

 the percentage of cerium oxide being larger than that from any known 

 igneous rock, with the exception of the nepheline-syenite from 

 Almunge in Sweden. 



The author has calculated the norm from the old and the new 

 analyses, and finds that the rock falls into the subrang rockallose 

 with the general symbol III. 3. I. 5. He notes, as indicating the 

 exceptional character of the rock, that these analyses are the only 

 representatives of the subrang rockallose among the 8,000 analyses of 

 igneous rocks that he has now collected. 



The author concludes with a discussion of the affinities of rockallite, 

 and a consideration of the probable chemical composition of the aegirite 

 and acmite. He clearly proves that the zirconia and cerium oxide 

 enter into the composition of the pyroxenes. 



II. MiNEEALOGICAL SoCIETY OF LoNDON. 



March 17, 19 14. -Dr. A. E. H. Tutton, F.R.S., President, in the Chair. 



F. P. Mennell : On an occurrence of Bornite Nodules in shale from 

 Mashonaland. The ore-body of the IJmkondo mine in South-East 

 Mashonaland consists of a bed of shale through which are scattered 

 nodules of bornite, most probably pseudomorphous after concretionary 

 pyrites. The enclosing I'ocks are of the same age as the Waterberg 

 Series of the Transvaal, and contain pseudomorphs after salt in some 

 of the shale bands. The occurrence of copper and salt at nearly the 

 same horizon is paralleled in the Lower Keuper beds of Europe. — 

 A. Scott : Augite from Bail Hill, Dumfriesshire. It occurs in crystals, 



