part 4] isroRiTE or sierra leone. 381 



In rare cases there is even seen a quaternary intergrowth 

 •composed of felspar, pyroxene, olivine, and magnetite. This inter- 

 growth is of considerable interest, because it contains all the 

 principal components of the rock, and is therefore practically the 

 theoretical eutectic of the magma. An almost parallel case has 

 been described by Dr. R. H. Rastall ^ from the Skiddaw Grranite, 

 in Avhich a quaternary eutectic of four of the five principal com- 

 ponents is sometimes developed. 



The following is a brief description of the more interesting 

 intergrowths of the Sierra-Leone norite and of corresponding 

 intergrowths in other rocks : — 



(i) Binary. — Plagioclase and augite form two types of 

 graphic intergrowth in the norite. The first type is obviously 

 primary, and occurs as narrow parallel rods of augite penetrating 

 the felspar (see PI. XVIII, figs. 5 & 6) ; a similar intergrowth has 

 been noted in the Norwegian syenite-pegmatites,^ in the coarser 

 parts of the Rowlej^ basalt,^ and in the Duluth gabbro.^ The 

 second t3^pe is of slightly later age, and generally takes the form 

 ■of a corona or reaction-rim partly enclosing biotite and olivine at 

 their contact with plagioclase ; it is developed only on the plagio- 

 ■clase side of the contact and spreads more or less fanwise into 

 the felsjmrs (see PL XIX, fig. 1). It is in general readily distin- 

 guished from the first type of intergrowth, both by its finer texture 

 iind b}^ its relation to other minerals, particularly the felspars. The 

 manner in which it is seen to spread through the felspars indicates 

 that, although formed after the main period of crystallization of 

 the rock, it was nevertheless connected in some way with the final 

 stages of consolidation. That it is unlikely to be a secondary 

 alteration-product is shown by the beautifully fresh condition of 

 all the minerals of the rock. With regard to the production of a 

 similar intergrowth in the Duluth gabbro, Nebel writes (o/:>. cif. 

 pp. 376-77 & pi. xiv, fig. h) :— 



' The evidence is not conclusive, however, as to whether the process was a 

 ■phase of the last stages of crystalHzation of the magma, or whether it 

 was a metamorphic process which took place after the rock was completely 

 solidified. The fact that the gabbro shows many metamorphic features 

 indicates that a metamorphic origin for the intergrowth is possible.' 



Dr. J. J. Sederholm clearly favoured the theor}^ that this inter- 

 growth was of a secondary nature, and formed by metamorphic 



1 ' The Skiddaw Granite & its Metamorphism,' Q. J. G. S. vol. Ixvi (1910) 

 p. 120. 



2 W. C. Brogger, ' Die Mineralien der Syenitpegmatitgange der Siidnorweg- 

 ischen Augit- & Nephelinsyenite ' Zeitschr. Krystallogr. vol. xvi (1890) 

 pp. 149-52. 



'^ J. J. H. Teall, ' British Petrography ' 1888, p. 212 & pi. xxiii, fig. 2. 



^ M. L. Nebel, ' The Basal Phases of the Duluth Gabbro, near Gabami- 

 chigami Lake (Minnesota) & its Contact-Effects' Econ. Geol. vol. xiv (1919) 

 p. 371 & pi. xiv, fig. a. 



Q. J.G. S. No. 312. 2 a 



