332 DR. T. DIXET OX THE [vol. Ixxviil^ 



agencies later than the solidification of the rock.^ In the Sierra- 

 Leone norite, however, the intergrowth is frequenth' deA^eloped in 

 places where it appears very unlikely that metauiorphic agencies, 

 have had any influence. It is, nevertheless, true that intergrowths 

 of felspar and augite are more abundant in the contact-altered 

 norite than in the imaltered rock (see below, p. 338 & PI. XIX,. 

 fig. 6), but this is due only to the increased op])ortunity of crystal- 

 lization afforded by the great heat of the intrusions, whereb}" 

 the two minerals, already present in eutectic proportions, could 

 recrystallize in graphic intergrowth. 



Plao'ioclase and olivine form relativelv rare intero-rowths. 

 that follow a crude graphic plan ; the small blebs and shoi-t laths 

 of olivine embedded in the felspar are in optical continuity with a 

 neighbouring larger crystal of olivine. Similar occurrences have 

 been noted and fio-ured br W. S. Bavlev- and M. L. Xebel.-^ These 

 minerals occasionally enter also into simultaneous crystallization 

 after the fashion of anorthite and olivine in the alliA'alites of 

 Eum.^ 



Olivine and magnetite form a graphic intergrowth recalling 

 the dendritic inclusions of magnetite in olivine. Intergrowths of 

 these minerals have been described from certain rocks in Alno.-^' 

 In highly ferriferous parts of the norite, augite and magnetite 

 may frequently be seen to bear similar interstitial relations to the 

 remaining minerals ; sometimes, in these circumstances, they entei^ 

 into a graphic intergrowth consisting of rods of the two minerals 

 lying parallel to the cleavage of the augite. Intergrowths of these 

 two miinerals have also been noted by Dr. A. Harker.^ Moreover, 

 hypers t bene and magnetite enter into intergrowth in the 

 form of plates of hypersthene enclosing graphic magnetite (see 

 PI. XIX, fig. 2). Intergrowths of the monoclinic and rhombic 

 pyroxenes have been described above (see p. 326). 



The following intergrowths also occur — (a) pyroxene and 

 olivine" ; (b) plagioclase and magnetite, in which rods and 

 narrow tongues of magnetite penetrate the felspar 8; and (c) in 

 the aplite-veins, hornblende and biotite. 



(ii) Ternary. — The most striking ternary intergrowth is that 

 composed of parallel rods of felspar, pyroxene, and magnetite 



^ '■ On Synantectic Minerals & Related Phenomena ' Bull. Comm . Geol. 

 Finlande, No. 48 (1916) pp. 9-46. 



^ ' The Basic Massive Eocks of the Lake Superior Region ' Journ, Geol. 

 vol. i (1893) p. 709. 



'^ Econ. Geol. vol. xiv (1919) p. 372. 



^ ' The Geology of the Small Isles of Inverness ' Mem. Geol. Surv. Scot. 

 1908, p. 88. 



"^ E, Workman, ' Calcite as a Primary Constituent of Igneous Eocks ' 

 Geol. Mag. 1911. p. 193. 



6 ' The Natural History of Igneous Eocks ' 1909, p. 271. 



'' Compare Yogt's researches on slags, 'Die Silikatschmelzlosungen ' pt. i 

 (1903) Vidensk. Selsk. Skrifter, Christiania. 



^ See also J. H. L. Yogt. ' On Labradorite-Norite with Porphyritic Labra-- 

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



