part 4] NORiTE or sierea leone. 333 



[C 68]. (See PI. XVIIT, fig. 6.) A second system, similar in 

 many respects to one seen in the Diiluth gabbro,i consists o£ 

 pyroxene, olivine, and magnetite. In certain of these intergrowtlis 

 minerals form angular and wedge-shaped masses, which irregularly 

 interlock and enclose one another. Interlocking rhombic and 

 monoclinic p3a'oxenes are occasional!}^ associated with one other 

 mineral in ternary intei'growth. 



(iii) The principal minerals of the quaternary intergrowtlis are 

 pyroxene, magnetite, and olivine ; felspar enters in as the fom-th 

 constituent, intergrown graphically with one or more of the other 

 minerals. These intergrowtlis are similar to the quaternary 

 eutectics noted by Prof. Lacroix in certain pyroxene-gneisses. ^ 

 Intergrowths of this order are sometimes formed also from the 

 ternary intergrowths of the norite, through replacement of a simple 

 pyroxene by intergrown rhombic and monoclinic pyroxenes.^ 



It is possible to express in a simple diagram the variable minera- 

 logical composition of the norite and also the development of the 

 ternary systems from magma of a given composition. It v^dll be 

 assumed in that part of the diagram which illustrates the sequence 

 of crystallization in the ternary systems, that the crystallization 

 proceeds without interruption or modification due to the formation 

 of mix-crystals. This assumption is justified, with the minor 

 exceptions of olivine and pyroxene taking up small amounts of 

 magnetite and titanic oxide respectively, and of felspar absorbing 

 small quantities of other minerals.^ Substances held in solid 

 solution in this manner tend to be precipitated on the cooling of 

 the rock, and to assume the form of minute inclusions. 



Suppose that the oxides SiO.^, Fe.P3.FeO, CaO, MgO, Na.O, con- 

 stituting the principal chemical components of the norite-magma, 

 are placed at the angles of a pentagon (see hg. 2, p. 334). The 

 area, as a whole, represents the general composition of the magma ; 

 the point SiO^ represents the mineral quartz, and the points A]^0^ 

 and re.p.3.FeO, represent the minerals alumina and magnetite 

 respectively ; the areas CaO, Aip.,, SiO^, and MgO, Fe.Pg, SiO.^,. 

 felspar and olivine respectively ; whereas the central part of the 

 pentagon represents pyroxene. Now take a felspar, magnetite, and 

 a pyroxene represented b}^^ the points A, B, C respectively ; any 

 point P within the area ABC will represent a combination of 

 these three minerals, in proportions which are inversely as the 

 distances of the point P from the points A, B, C. The composition 

 of the magma representing the tern aiy eutec tic of the felspar, mag- 

 netite, and p3''roxene can then be found as follows : — Erect ordinates 



1 M. L. Nebel, Econ.^Geol. vol. xiv (1919) p. 372. 



2 ' Contributions a I'Etude des Gneiss a Pyroxene & des Roches a Wernerite '' 

 Bull. Soc. Min. France, vol. xii (1889) p. 83. 



•^ See J. H. L. Vogt, ' On Labradorite-Norite with Porphyritic Labradorite- 

 Crystals, &c.' Q. J. G. S. vol. Ixv (1909) p. 81. 



4 See J. V. Elsden, 'Principles of Chemical Geology' 1910, p. 182. 



2a2 



