Petrography. — "Oti the Rocks of Doonnantop in Central 

 New Guined" . By W. F. Gisolf. (Communicated by Frof. 

 G. A. F. Molengraaff). 



(Communicated at the meeting of February 24, 1923). 



During a caiLserie about New Guinea, delivered at Batavia, Dr. 

 H. J. Lam of Buitenzorg, at a meeting of the "Koninklijke Natuur- 

 kundige Vereeniging", showed a sample of a rook from Doormantop, 

 which directly engrossed my attention to such an extent tliat I 

 asked him to leave it to me for examination. He readily did so. 

 Afterwards he furnished me with more samples of the same material, 

 for which kindness I hereby tender him my best thanks. The geol- 

 ogist of the Mamberamo-expedition Dr. P. F. Hubrkcht, was staying 

 in East-Java at that time, and was not in a position, within the 

 first ten months, to send me any material. However, when asked, 

 he did not object to an examination of the samples nor to public- 

 ation of the results. I have much pleasure in thanking him also 

 for his kindness. 



The first .samples that came to hand, present a schistose structure, 

 chiefly due to parallel bands of magnetite; they are of a dark green 

 colour, with a thin ligiit-brown non-detached weathered crust of a 

 cavernous appearance, on either side a relatively considerable 

 quantity of magnetite reveals itself in non-crystallized masses; the 

 erosion has spared the magnetite, so that it projects '/, — 1 centim 

 from the rock. A blow of a hammer made the rock split along the 

 magnetite, thus effecting the first separation between the rock and 

 the ore. 



Some slides were made of the part from which the magnetite 

 had been removed as nuicli as possible. Under the microscope the 

 rock proved to consist of magnetite with fresh olivine and a 

 colourless, lath-shaped mineral of moderate refringence and very- 

 weak birefringence. The structure is slightly varying, the olivine now 

 encloses tlie colourless mineral, now it mingles with it as if they 

 were crystallized out simultaneously ; the magnetite encloses the 

 colourless mineral and occurs xenomorphic in the aggregate olivine- 

 unknown material. The magnetite is polarimagnetic. A little apatite 

 presumably occurs. • 



