486 Transactions. — Geology. 



Art. LV. — On the Cause of Border - segregation in some 



Igneous Magmas. 



By Professor James Park, M.A.Inst.M.B., M.Inst. M.M., 

 F.G.S., Director Otago University School of Mines. 

 [Read before the Otago Institute, 13th September, 1904.] 

 It has been ascertained by microscopic investigation that 

 nearly all igneous rocks vary in composition in different parts, 

 due to the aggregation of certain minerals in clusters, which 

 vary in size from the smallest dimensions up to large rock- 

 masses. Dr. W. C. Brdgger and Professor J. H. L. Vogt 

 were the first to direct attention to the fact that igneous 

 rocks are generally more basic near their borders than in the 

 interior of the mass. It is found that the heavier minerals 

 that first crystallize from the magma — namely, magnetite, 

 olivine, chromite, hornblende, pyroxene, &c. — are principally 

 concentrated in the outer portions of the igneous body. 



Many valuable deposits of magnetite and chromite occur 

 as border-segregations, and for this reason much discussion 

 has taken place among mining geologists as to the force or 

 energy concerned in the migration of these minerals from 

 one part of the magma to another. The subject is one having 

 a profound bearing upon magmatic differentiation generally. 



For years it was commonly believed that border-aggregation 

 was due to molecular flow, in accordance with Soret's prin- 

 ciple that molecular concentration may be caused by differ- 

 ence of temperature— that is, in a homogeneous solution 

 unequally heated in different parts concentration will be 

 greatest in the region of lowest temperature. 



A. Harker ,: and G. F. Beckerf have contended that mole- 

 cular flow in a cooling magma is too slow in its operation 

 to be the cause of magmatic differentiation. And more lately 

 Becker] and T. E.Spurr,§ simultaneously and independently, 

 have argued that the migration and concentration of the first 

 crop of heavy minerals near the borders, to a greater or less 

 extent, may be due to convection currents resulting from 

 differences of temperature. But even this hypothesis does 

 not seem quite satisfactory. In the case of an eruptive magma 

 in finite mass confined between walls, the direction of the 

 convection currents in the central portion would be upward 



* A. Harker, Quart. Journ. Geol. Soc, London, vol. i., 1891, p. 311. 



fG. F. Becker, "Some Queries on Rock Differentiation," Amer. 

 Journ. Sci , Jan. 1897. vol. in., p. 21. 



I G. P. Becker, " Fractional Crystallization of Rocks," Amer. Journ. 

 Sci., Oct., 18 1 J7, p. '257. 



§ J. E. Spurr, "Igneous Rocks as related to Occurrence of Ore," 

 Trans. Aust. Inst. Ming. Eng., vol. xxxiii., 1903, p. 288. 



