Pakk. — Magmatic Segregation. 13 



sent the laws governing magmatic differentiation are but im- 

 perfectly understood. By some border segregation is ascribed 

 to molecular flow due to differences of temperature in the 

 magma ; by others to convection currents, which it is believed 

 would tend to carry the first crop of minerals, such as magne- 

 tite, olivine, &c, to the borders of the igneous magma. 



The writer* is inclined to ascribe border segregation to the 

 difference of osmotic pressure that must exist in a finite mass 

 of magma cooling more rapidly in the borders than in the cen- 

 tral portion. 



The valuable ores that may be considered primary con- 

 stituents of eruptive rocks, resulting from direct differentiation 

 in the cooling magma, are as follows : — 



(a.) Chromite in peridotite and serpentine. 



(b.) Copper and nickel-iron in serpentine. 



(c.) Platinum metals in highly basic eruptives. 



(d.) Magnetite and titanite in basic and semibasic eruptives. 



Chromite in Peridotite. 



In the South Island of New Zealand there are two moun- 

 tain-masses of peridotite in which the magmatic segregation 

 of chromite is exhibited on a scale of unusual magnitude. 



A few miles from the City of Nelson, Dun Mountain rises 

 to a height of over 4,000 ft. above sea-level. It covers an area 

 of about four square miles, and is entirely composed of massive 

 olivine, in which chromite of iron is fairly uniformly dissemin- 

 ated in the form of fine grains, but is occasionally aggregated 

 in large masses. f The adjacent rocks are slaty shales and 

 limestone of Jurassic age, the limestone occurring at the 

 base of the sedimentary formation. Between the limestone 

 and the olivine, to which HochstetterJ gave the distinctive 

 name " dunite," there is a belt of serpentine, half a mile 

 wide. The serpentine contains lenticular-shaped masses 

 of chromite, native copper and copper-ores, principally 

 chalcopyrite, with the usual products of oxidation. It also 

 contains thin irregular veins of diallage, hypersthene, bronzite, 

 enstatite, scapolite, wollastonite, and chrysolite. The larger- 

 deposits of chromite occur near the borders of the olivine and 

 serpentine. 



The second great mass of peridotite forms Bed Mountain, 

 situated twenty miles north of Milford Sound, in Otago.S It 



* J. Park, "On the Cause of Border Segregation in some Igneous 

 Magmas," Trans. N.Z. Inst., vol. xxxvii, 1905. 



f S. H. Cox, "Chrome-deposits of Nelson," New Zealand Geol. 

 Reports and Explorations, 18S1, p. 8. 



J Dr. F. von Hochstetter, Zeitschrift der Deutschen Geol. Gessell- 

 schaft, vol. xvi, p. 341. 



§ J. Park, N.Z. Geol. Reports and Explorations, 1886-87, p. 121. 



