Vol. 55.] AND FORSTEKITE FEOM THE GLENELG LlilESTOlSrE. 379' 



The specific gravity of each mineral was determined in a pycno- 

 meter with the purest samples, and the results were : — for spinel, 

 3-57 ; for forsterite, 3-24. 



The results of the analysis of the spinel (weight of substance 

 =•391 gramme) are : — 



SiOa Silica 1-20 



AI2O3 Alumina 69-80 



FeO Ferrous oxide 2-03 



MgO Magnesia 2730 



100-33 



Or, SiO^ : Al^Og : (FeMg)O. 

 •03: i : 1-04 



The results of the analysis of the forsterite (weight of substance 

 = •500 gramme, and for FeO '256 gramme) are : — 



SiOa Silica 41-16 (trace of TiO^) 



AI2O3 Alumina 1-02 



FeO Ferrous oxide 2*00 



CaO Lime -26 



MnO Manganous oxide '26 



MgO Magnesia 54-86 



Loss on ignition '70 (trace of F) 



100-26 



Or, SiO^ : (Mg, Fe, Ca, Mn)0 : Al.fi^ 

 •687: 1-411 : -01 



The small amount of alumina in the forsterite is in all probability 

 due to a trace of spinel. 



In conclusion, we wish to express our great obligation to our 

 colleague, Mr. J. J. H. Teall, F.E.S., who has determined the 

 minerals in the thin slices to which we have referred, and has also 

 helped us in other ways. We have also to thank Mr. Pringle, 

 of the Jermyn Street Museum, and Mr. Prior, of the British 

 Museum (Nat. Hist.), for searching out specimens to compare with 

 those ^from Glenelg. The spinel of Glenelg closely resembles that 

 from Aker in colour and mode of occurrence, but in the latter 

 spinel crystalline form is^better developed. One specimen in the 

 Ludlam collection from Aker bears a striking resemblance to the 

 Glenelg specimens, and shows the spinels embedded in a limestone 

 which also contains phlogopite and diopside. 



Attention should, moreover, be called to a paper by E. Weinschenk,* 

 in which he describes various minerals, including forsterite, spinel, 

 and phlogopite, that occur in the limestones accompanying the 

 graphitic beds north-east of Passau. As already stated, there are, 

 near the Glenelg limestones also, various bands of rock which are 

 rich in graphite. 



^ ♦ Beitrage zur Mineralogie Bayerns,' Zeitschr. f. Kryst. vol. xxviii (1897) 

 p. 315, and reviewed in Neues Jahrb. 1898, vol. ii, p. 20. 



