Dr. F. P. Mueller — Teldite from British Borneo. 209 



Physical properties. — The specific gravity is G = 2"457. The 

 hardness is H = 6. The colour is dark-green brown. The index of 

 refraction, measured on two optic prisms prepared from No. 2, was 

 found n— l - 5097 (Na). All sections revealed without aid of the 

 microscope a fluidal structure. 



Microscopic examination proved the specimens to be an almost 

 pure glass. A very few, small, scattered vesicles were noted, 

 while by aid of the strongest enlargement only a few indeterminable 

 mineral particles were observed. Comparing the physical properties 

 with those of Billitonites it is evident that they are almost identical. 



Chemical composition. — The following analysis of a Brunei tektite 

 was made by Dr. Hinden in Basle. In the first column is shown the 

 percentage of each oxide, and in the second is given the molecular 

 proportion. 



Table II. 



The accompanying diagram (Fig. 2), p. 210, has been drawn in order 

 to compare the Brunei tektite with the Billitonites and the best 

 corresponding Australites. It shows the molecular proportions of 

 the principal oxides plotted as ordinates, those of the silica taken 

 as abscissae of five analyses of Australites (Nos. I, IV, V, YI, YIII), 

 two of Billitonites (Nos. 9, 10), and one of the Brunei tektite 

 (No. a). The diagram is a modification of the basic portion of F. E. 

 Suess' Table iii, 1914. The roman numerals correspond with those 

 by which H. S. Summers (Proc. Boy. Soc. Victoria, vol. xxi, 1908) has 

 marked the analyses of the Australites. The Arabic numerals are the 

 numbers given in the same analyses as well as the analyses of the 

 Billitonites by F. E. Suess, 1914, pp. 86-7 (see Table III, p. 211). 



The examination of the diagram shows that the Australites and 

 the Billitonites form two well-marked groups, their chemical 

 variation being different in character. F. E. Suess (1914, p. 98) 

 states as characteristic of the Billitonites the small percentage of 

 alumina and the high percentage of alkalies. The rapid decrease 

 of iron, magnesia, and lime is equally striking, while for the 

 Australites these substances maintain an almost uniform position, 

 diminishing very slightly only towards the acidic end of the series. 



The Brunei tektite, as expressed by the diagram, though containing 

 an almost equal quantity of silica as the Billitonites, shows little 



DECADE VI. — VOL. II.— NO. V. 14 



