CLASSIFICATION OF IGNEOUS ROCKS 225 



(3430) Several leucitites, so called, fall here, although they are 

 not true leucitites on account of their feldspar content. No 

 plutonic rocks have been located. 



CLASS 4, ORDERS I TO 3 



No attempt is made in this paper to separate the orders of Class 4, 

 since too few modes have been found in the literature to warrant 

 the separation, and measurements by the author, so far, have been 

 chiefly on the rocks of the first three classes. It is hoped shortly to 

 give the modes of some of the classic types of Class 4. 



Family i. — Dunite von Hochstetter. This rock, named by 

 von Hochstetter^ from the Dun Mountains, New Zealand, consists 

 of olivine with accessory chromite. The amount of chromite varies 

 greatly. Vogt^ says that the normal rocks carry from 2 to 5 per 

 cent. They would thus belong to Order i . Dunites with between 

 5 and 95 per cent chromite he calls chromite-dunites. With over 

 95 per cent chromite the rocks may be classed as chromite ores 

 of Order 4. They are represented by the plagioclase-free varieties 

 of Sjogren's^ Kromit-Olivinit. In other cases the ore is magnetite, 

 as in the magnetite-olivinite series of Sjogren, which includes 

 plagioclase-bearing and plagioclase-free olivine-magnetite rocks. 

 Perhaps good subdivisions would be: 



Order i. Dunite. Olivine between 100 and 95 per cent. 



Order 2. Chromite-dunite and magnetite-dunite. Olivine between 95 

 and 50 per cent. 



Order 3. Olivine-chromitite and olivine-magnetitite. Olivine between 

 50 and 5 per cent. 



Order 4. Chromitite and magnetitite. Olivine less than 5 per cent. 



Families 2 and 5. — These are the families of the mica- 

 (amphibole-) olivine rocks. Among them are mica-peridotite and 



'Ferdinand von Hochstetter, "Dunit, korniger Olivinfels vom Dun Mountain 

 bei Nelson, New-Seeland," Zeitschr. d. d. geol. Gesell., XVI (1864), 341. 



^ J. H. L. Vogt, "Beitrage zur genetischen Classification der durch magmatische 

 Differentia tionsprocesse und der durch Pneumatolyse entstandenen Erzvorkommen,"' 

 Zeitschr. f. prak. Geol. (1894), p. 391. 



3 A. Sjogren, "Om forekomsten af Tabergs jernmalrtisfyndighet i Smaland," 

 Geol. Foren. i Stockh. Forhandl., Ill (1876), 58. 



