Geology. — "On the occurre^ice of diamond ns an accessory mineral 

 in olivine and anorthite bearing bombs, occurring in basaltic 

 lava, ejected by the volcano Gunung Ruang {Sangir- Archipelago 

 north of Celebes)." Bj Dr. W. F. Gisolf. (Comminiicated 

 by Prof. EuG. Dubois). 



(Communicated at the meeting of .Tune 30, 1923). 



Dr. G. L. L. Kemmekling, chief of the volcanological survey 

 of the Dutch East Indian Archipelago, liaving collected some 

 bombs ont of the basaltic lavas from the volcano Gunung Ruang, 

 composed of a mixture of a dark green to black mineral and glassy 

 plagioclase, the latter in crystals of a size np to 1 cm., kindly 

 intrusted those to the author for microscopical examination. 



Two kinds of rock were collected ; the lirst of these is dense 

 and black and shows strong magnetic properties; examination with 

 a magnifying glass reveals the presence of magnetite with a tinge of 

 blue; under the microscope it proved to be composed of densely 

 crowded grains of magnetite and between those one can indistinctly 

 recognize strong pleochroitic hypersthene and green coloured mono- 

 clinic pyroxene. 



The second kind, which contains much less magnetite, is com- 

 posed of corroded olivine, fringed by a border of strong pleochroitic 

 hypersthene; it also contains bottle-green monoclinic pj'roxene. The 

 plagioclase in both kinds of rock could be determined as belonging 

 to members of the group which are very rich in anorthite. 



In some of these rocks an accessory mineral occurs in a con- 

 siderable number of minute grains. They may be seen most clearly 

 in specimen 285 A in which the olivine can be recognized macro- 

 scopically ; around the bomb a crust of the lava in which it is 

 imbedded is to be seen; this lava lias the composition of a basalt 

 with bottle-green augite and very basic plagioclase. 



The plagioclase is twinned according to the albite- and Carlsbad 

 laws; one of the thin sections shows a plagioclase with three lamellae; 

 the first shows in the conoscope, between crossed nicols, the emerging 

 of the Z-SiX\s within the field of view, slightly inclined to the surface 

 of the section ; the extinction amounts to 65 degrees. The second 

 lamel shows in the conoscope the emerging of the .Y-axis, also 



